A component that plugs into the output side of the device that happens to be 'ahead' (eg FM radio) of its desired simulcast partner (eg digital TV), comprising a rotary dial and a bucket-brigade delay chip.
The viewer wishing to watch and listen with vision and sound synchronised introduces a delay (set by twiddling the dial appropriately) to their radio output such that the audio is then in sync with the video being received by the TV.
Fiver a time, bish-bosh, problem solved.
You should be able to buy them directly off the BBC website, as well as in every consumer electrical outlet. Heck, it's cheap enough to build that the BBC could give 'em away on demand.
I'd call it the 'Simulcastermaster' :-)
NB: This assumes the most probably situation is that the audio is leading the video - which could, in fact, be engineered at source to always be the case. In the event that the video is leading the audio, then I'm afraid we need 'slow glass' :-)