A flinty government vetting officer turns up at the British Embassy in Tazbekistan. He's so tough he walked from the airport 15 miles away and is barely perturbed by the fact that the country is in the grip of revolution.

As the power fails and shots ring through the night air, British ambassador Keith Davis and his deputy Neil Tilly (David Mitchell and Robert Webb) have to think on their feet. Someone has to suck up to the president's monstrous daughter, a primped, plastic surgery-obsessed brute with her own wine label; while Keith has to make peace with the insurgents, though they are late for a rendezvous: "You can't expect punctuality from rebels," he says with true British grit. "They are rebels, aren't they?"

It's been an engaging series; I hope it returns.

Where, exactly, has Ambassadors gone wrong? The idea, sending blundering diplomats to a chaotic post-Soviet state, is sound, while the casting - with Mitchell & Webb as inept and smug respectively - is tried and tested. The fault lies in the tone: Ambassadors demonstrates, like so many others before it, just how difficult a comedy-drama can be to execute.

Tonight's final episode is another slice of ripe characterisation, featuring Michael Smiley's crack government interrogator and Natalia Tena as the Tazbek President's spoilt, tarty daughter. There are promising situations too, as a power cut and an ill-advised tryst leave Keith (Mitchell) and Neil (Webb) looking distinctly compromised. But the whole simply doesn't hang together - infuriating, given the talent involved. Ambassadors may need a rethink before serving another term.

Ambassadors is the low-key acerbic comedy drama set in a British embassy in the fictional central Asian country of Tazbekistan. The first episode had some tonal problems as it struggled to establish whether it wanted to be funny or clever, and often failed to achieve either.

But it was OK, and at times mildly amusing, which already put it out in front of most of the competition. That said, you expect better than occasionally mildly amusing from David Mitchell and Robert Webb, who maintained a level of demented brilliance in Peep Show for years.

And in the second episode they were indeed much better. Some of the improvement could be attributed to a wonderful turn by Tom Hollander as an obnoxious prince who stumbles luxuriously around the globe as a trade envoy creating international crises - a great comic idea, and one wonders who could possibly have been its inspiration.

More than that, though, it was a matter of characters falling into place and the place finding its character. Webb is oddly convincing as a cynical idealist assistant to the ambassador, and Mitchell shows a conflicted steeliness and sensitivity that goes some way beyond his stock gift for the florid rant.

The writing, by James Wood and Rupert Walters, was sharper too. Several plot strands were neatly combined, and there was an impressive resistance - as shown with the Prince Mark storyline - to succumbing to the obvious. Rather than bash you over the head with jokes, it takes a more diplomatic approach. And I don't care what Steve Coogan says about him, Mitchell has persuaded me on this one.

In this amusing second of a three-part comedy-drama, British ambassador Keith Davis (David Mitchell) and his No 2, Neil Tilly (Robert Webb) face the prospect of a "diplomatic" visit from a member of the royal family (Tom Hollander). What follows is very funny, but as much as it is a modern political situation (it's set in "Tazbekistan", a former Soviet republic) with some amusing satirical points (bribes for contracts, royal faux pas), this is basically Mark and Jez in suits, performing some effective but ultimately traditional comedy.

Tom Hollander (Rev.) steals the show from under the noses of David Mitchell and Robert Webb tonight, stiffening his upper lip as minor British royal Prince Mark. A stroppy chap, whose lot in life is to serve as a trade envoy, the prince is summoned to Tazbekistan by Ambassador Keith (Mitchell) who's trying to schmooze another commercial deal out of the Tazbeks - and they do love a good royal. But, as it turns out, the petulant prince has a few surprising diplomatic talents up his sleeve.

Rev. star Tom Hollander effectively steals the show from under the noses of the two leads when he guests as Prince Mark, a minor royal who's been invited to Tazbekistan to grease the wheels for another trade agreement.

Prince Mark, we learn, always travels with a 6ft ironing board carried by his loyal male assistant called Treasure.

This is the easy stuff and it's a shame Ambassadors isn't content to just go straight for the comedy jugular.

Goodness knows it's got all the right ingredients in Robert Webb and David Mitchell, while the two secret police spies eavesdropping on all their conversations are sweetly hilarious.

Instead we're asked to wade through storylines about escaped human rights protesters, enforced child labour and threats to murder women and scatter their chopped up remains in the woods.

Things like that do tend to wipe the smile right off your face.

Tonight's opening scene in which Neil Tilly (Webb) is kidnapped and a sack pulled over his head - for laughs - is an example of the impossible balancing act Ambassadors has set itself.

The British Embassy in Tazbekistan prepares for the arrival of Prince Mark, a trade envoy and minor member of the royal family. He turns out to be a spoilt, entitled berk (a great comic turn from Tom Hollander) who travels everywhere with a 6ft ironing board so his factotum can ensure the creases in the Prince's trousers are perfect.

Though Ambassadors feels a shade underwritten and falls uncomfortably between drama and comedy (that tricky middle ground is always a hard place to sit), it bowls along thanks to a sharp cast - David Mitchell and Robert Webb play the ambassador and his capable deputy, and there's a great foul-mouthed cameo from Matthew Macfadyen as their furious government boss.

And Frasier fans, watch out for Edward Hibbert (restaurant critic Gil Chesterton) as an equerry.

While last week's tepid opener failed to prove either way whether Ambassadors could make foreign affairs funny, it's case closed by the end of this direly unfunny middle episode.

Tonight sees British ambassadors Keith (David Mitchell) and Neil (Robert Webb) forced to babysit a visiting prince (well played by Tom Hollander), while negotiating oil-drilling rights with the premier of their fictional Tazbekistan. Hilarity does not ensue. Aimed squarely at fans of The Thick of It, Ambassadors delivers precisely none of the nuanced comedy and Kafka-esque scenarios.

Given the highbrow subject matter, there's an awful lot of physical daftness, including a tiresome skit in which Hollander's prince berates a blind person while - wait for it! - not realising he's blind. The central duo don't add much, either. It's as if writers Rupert Walters and James Wood had hoped the Peep Show boys would ad-lib all the funny bits, only to find out they couldn't be arsed. By the end of this, neither will you be.

Ambassadors pokes fun at Prince Andrew

A BBC comedy will this week take a dig at Prince Andrew for his globe-­trotting role promoting Britain.

Plunging bravely into a ethical faultline was BBC2's Ambassadors, which is about foreigners in a little-known Central Asian nation. Borat! I hear you exclaim. But this three-part comedy drama is not a politically incorrect gag fest. It stars the comic duo David Mitchell and Robert Webb, and appears to be aiming to fit in some thoughtful messages about foreign policy. Mitchell plays the British ambassador to a sandy republic called Tazbekistan who has to deal with arms and business deals, desert despots, self-absorbed western activists, and peculiar local customs. Webb plays his diplomatic deputy who's engaged in dodgy sidelines.

The first episode struggled for a consistent tone. Mitchell appeared to be the only funny man in the show - by which I don't mean that he was the only actor who managed to make us laugh, but that he was the only person who seemed to be starring in an absurdist comedy. The other actors inhabited a straight-up drama that just happened to have amusing lines in it. Mitchell's character romped around the wilds of Tazbekistan hunting, accidentally shooting the mighty horned ibex, the beloved national animal. His pudgy face peeking out from under a cartoony deerstalker, he acknowledged looking like Elmer Fudd. The rest of the cast delivered wisecracks, but in the manner of, say, a smart-ass detective show. Webb's worldly character, with his blackmail worries, wouldn't have been out of place in a crime thriller.

Genres jostled with each other like expats at the bar of a far-flung Irish-style pub. The show's desire for mild moralising also collided with its own premise. Yes, the programme is about consuls in an exotic locale, but we can't have too many jokes about weird local practices, so let's also make fun of British stereotypes. The result is a half-hearted scene of the Tazbek president carving up an ibex carcass and another of an imported English pork-pie festival. In the mix is some chit-chat about the deadly aspects of British helicopter sales. Still, I'm happy to say the second episode, which features a (fictional) royal dignitary, is more surefooted. Mitchell's ambassador becomes less spoofy, and the programme gets into its stride of making points about the farcical nature of international relations.