ITV is pushing to invest in primetime comedy again, with loads more Benidorm in the pipeline, as well as two brand-new sitcoms tonight (see also The Job Lot). And they couldn't be more different in style. Vicious is the more old-fashioned - studio audience, huge sitting-room with front door, left, and swing-door to kitchen, right (it's The Golden Girls' format) - except that, in a très moderne move, the central figures are two gay guys.

Actorly ham Freddie (Ian McKellen) and swishy Stuart (Derek Jacobi) have been cattily in love for 48 years. While activists might cavil over stereotyping, there's no denying that the spectacle of two of our finest knights of the theatre camping it up is absolutely hilarious. Along with Frances de la Tour as their voracious mate, Violet, they make every line a zinger. Creators Gary Janetti (Will & Grace, Family Guy) and Mark Ravenhill (fruity West End plays) have a sure-fire hit on their hands.

Relishing the chance to camp it up, old queen style, thespian legends Ian McKellen and Derek Jacobi team up as delightfully tart couple Freddie and Stuart for this theatrical new sitcom. There's something of the retro spirit of Rising Damp about Vicious, with its waspish wit and twilit interior - the fabulous Frances de la Tour is even on hand as fag hag Violet. Tickling the trio's sensibilities is young flat-hunter Ash (Iwan Rheon) who stirs Freddie's gaydar - which clearly needs a 21st-century upgrade - and to get the party started there's a wake in honour of an old friend who, naturally, had a massive crush on the self-obsessed Freddie.

Ian McKellen and Derek Jacobi have a ball as a bitching couple living in a cobwebbed, sepulchral flat, lusting after hunky new neighbour Iwan Rheon, confiding in best friend Frances De La Tour and hamming it up wherever possible. It's a very traditional studio sitcom setup, made watchable by its stars and enjoyable by a waspish script. Also, in its combination of old age and homosexuality, it could be argued to have broken a little ground. Not that creators Mark Ravenhill and Gary Janetti much care about that: this show is all about low blows and easy laughs - at which it excels.

Derek Jacobi and Ian McKellen are Stuart and Freddie, a bitchy, bitter couple holed up in their dingy London flat. In this first episode, the pair become fixated with a handsome young neighbour, as does their brazen friend Violet (Frances de la Tour). With all the very loud talking and theatrical self-pity, this could have been a superbly camp sitcom, but the characters aren't nearly outrageous or monstrous enough. That is clear from their pedestrian put-downs: "At least I'm not from Leytonstone," being Freddie's stock comeback.

The humour may not exactly be top drawer, but the cast list of ITV's latest stab at primetime comedy can't be argued with. Sir Ian McKellen - Gandalf! Magneto! - and Sir Derek Jacobi - Claudius! The Master! - together... surely it can't be that bad?!

The Sirs play geriatric gay couple Freddie and Stuart, who in this opening episode decide to throw a camp and catty wake for their recently deceased friend. Gurning Misfits actor Iwan Rheon also appears as sprightly leather jacket fan Ash, who's just moved in upstairs, while another acting heavyweight Frances de la Tour plays the couple's ravenous friend Violet. The jokes are groan-worthy, clichéd and occasionally offensive, so we don't expect everyone to come away from Vicious having loved it, but legends like McKellen and Jacobi cosying up together on the same sofa just has to be seen...

A studio-bound, single-set, multi-camera sitcom, Vicious is a gratifyingly old-school farce in which thespian deities Sir Ian McKellen and Sir Derek Jacobi have a char-grilled whale of a time as an incessantly bickering homosexual couple. Sealed within their sepulchral Covent Garden abode - they shriek like vampires when the curtains are accidentally opened - pompous actor Freddie (McKellen) and retired bar manager Stuart (Jacobi) tussle waspishly over decades of perceived slights, while never missing an opportunity to mock each other's supposed decrepitude.

Now, these are hardly original comic creations - the vituperative, hammy old queen has long been a staple of popular culture - and there is nothing especially notable about the premise. But that simply doesn't matter when the execution is as strong as this.

Resembling a startled, wounded guinea pig, Jacobi squeals and frets amidst a knowing flurry of camp mannerisms, while McKellen booms fresh insults in that oak-lined voice of his. He also pulls some of the funniest "Why, I've never been so insulted in my life!" expressions this side of imperial phase Frankie Howerd. It's an impeccable dual assault of seasoned comic timing.

Enjoyment is magnified by the addition of Frances de la Tour as their dotty, man-hungry pal. Famously, she starred in Rising Damp, one of ITV's few great sitcoms, and it's tempting to view her presence here as a deliberate nod to the past. Not that her involvement is merely symbolic - she's a peerless comic actress - but you could argue that she's essentially playing lonely Miss Jones 30 years on. Even the dingy brown set recalls her most celebrated role.

Broad and boisterous in the best possible sense (ie it's nothing like that avalanche of horror, Mrs Brown's Boys), Vicious is jam-packed with gags, hitting the ground running with an impressive opening episode which establishes set-up, character and backstory with consummate ease.

A co-write between acclaimed playwright Mark Ravenhill and Gary Janetti, a former executive producer on Family Guy and Will & Grace, it revels in its camp bluster with such benign relish, I doubt it'll get into too much trouble for reinforcing stereotypes. It's obvious that Freddie and Stuart are blissfully happy in their enmity, and it's that undercurrent of warmth - the spoonful of sugar beneath the barrel-load of bile - that make these characters so engaging.

I'm no soothsayer - I've never said "sooth" in my life - but I predict that Vicious will be huge. A hit sitcom! On ITV! Nurse, the smelling salts...

Can you believe they managed to get Sir Ian McKellen and Sir Derek Jacobi on a TV show together... and then forced them to make awkward '70s sitcom-esque gags? Nah, we kid. Vicious seems like good fun, and we'll be tuning in just to see how wide Iwan Rheon can get his mouth to open. Seriously, in the trailer his jaw is flopped down like a Sesame Street puppet.

Last Tango in Halifax (BBC One) was very simply plotted, although, in contrast to a drama like The Town, writer Sally Wainwright's dialogue tended towards the emotionally incontinent.

"Why can't you accept that I want to be with Kate, that I am old enough to make my own decisions, and to accept it and be civilised about it?" wailed Sarah Lancashire's lesbian headmistress Caroline as she tried to drag her mother Celia (Anne Reid) into the 21st century.

A few scenes later and Celia had had a change of heart, declaring that she had "played hardball long enough" (perhaps the only 75 year-old in Britain to have ever used that term) and was ready to accept her daughter "for who she was" having been "on the road to Damascus".

Last Tango in Halifax was sometimes silly and two episodes too long. But it was that difficult beast, a comedy drama, and for all its faults could sometimes be both funny (any scene involving Tony Gardner as John, Caroline's feckless, needy ex-husband) and dramatic (any scene involving Derek Jacobi's Alan).

Pensioner Alan's late-blossoming courtship of Celia, his first love, was touching and the power came from Jacobi's understated performance. This theatrical knight has played many fascinating, complex men (Richard II, Francis Bacon, Alan Turing) but I have never seen him play ordinary. Often, great actors fail when they try to be the everyman; thwarted by their own heavyweight presence. But Jacobi as Alan achieved much by doing very little. Just by sitting in front of the Aga and sipping his tea thoughtfully, he vividly portrayed a kind, unremarkable man who had looked at life from atop his West Yorkshire farmhouse, and after three quarters of a century, had worked out its deepest mysteries. Amid the drama's silly theatrics of cheating spouses, concupiscent toyboys and alcoholic screw-ups, Jacobi added some much-needed depth.

The series has been a ratings success and will return next year when the grand and sometimes unlikeable Celia will prepare to walk down the aisle to The Entrance of the Queen of Sheba. Let's hope no pesky TV producer introduces an unfathomable story arc that will prevent her from getting her wish.

Last Tango In Halifax was a drama with heart to spare

It's not the sort of show that wins awards but it would be a crime if Anne Reid and Derek Jacobi didn't get the chance to dust off their thank you speeches on the back of Last Tango In Halifax (BBC1).

Having learnt that her daughter Caroline is a lesbian, Celia is shocked and judgemental. "What will folk think?" she demands of her fiancé, Alan. They'll be "pointing and saying things".

As Sally Wainwright's clever, multilayered drama ends, what started out as the most dreamy of courtships appears to have hit some very choppy waters when Alan (Derek Jacobi) decides he doesn't like this new side to Celia (Anne Reid). Caroline (Sarah Lancashire) isn't enamoured of it, either: "You're going to die lonely and bitter," she yells at her mother when the pair trade cruelties in a terrible row.

I'm going to miss Last Tango in Halifax, as will presumably its huge BBC1 audience. It was one of those dramas that arrived out of nowhere to an instant embrace from viewers, and none of us will quite be able to let Alan, Celia, Caroline and Gillian get away from us.

By the way, you will cry at the end. Oh yes, you will.