the show was

Several twitterers immediately helped out by telling me the name of the programme. It was BBC1's Bang Goes the Theory - The Human Power Station. (Thanks @carboncoach @bigbuzzard @dc325 and @bridgetmck.)

The programme was broadcast 3 December 2009. In the documentary, one family, the Collinses (pic), didn't know it but they had their electrics supplied to them by a team of cyclists on exercise bikes in a warehouse. The Independent review describes the state of the cyclists:

As the day wore on, they dropped like flies: faces red, foreheads sweaty, eyes rolling, they yelled discouragement each time anyone got near a switch.