With It's Only A Theory, BBC4 seem to have taken the unusual step of commissioning a Radio 4 comedy before it's even had a 6.30pm slot. Devised by the reliable Andy Hamilton, whose career takes in Shelley, Not The Nine O'Clock News, Drop The Dead Donkey, Armstrong And Miller and Outnumbered, it sees scientific experts bring their hypotheses to the studio where they are subjected to an interrogation by Hamilton, his regular sidekick, panel-show favourite Reginald D Hunter and a special guest, this week being the blokey Clare Balding. Like most of those Radio 4 shows it is amusing rather than funny and the best lines in this ep are delivered by ageing expert Aubrey De Grey, but it is nonetheless well worth a look.