Test: I made my New Year observations from a 4th floor balcony in Clerkenwell, with a wristwatch on one hand and a glass of champagne in the other. That's a proper wristwatch with seconds, not one of those fashion bracelets that merely tells you the big hand is somewhere near the blob at the top. Other partygoers were present on the balcony at the same time, but none of them were involved in the experiment. I was able to make my observations during the gap between two particularly debilitating bouts of hiccups, a condition which beset me on several further occasions throughout the early hours of the morning.
From my position on the balcony I was able to see: a) the top quarter of the London Eye, poking out above the rooftops of central London b) a rather flashy widescreen (digital) television, tuned into 'New Year Live' on BBC1
As midnight approached I watched (on television) the 60 second countdown being projected onto the face of the Shell Building beside the Thames. I noted that the TV countdown appeared to be running one second behind the time on my wristwatch. Digital television was indeed running slow, as predicted. Big Ben proved to be inaudible both on the television and in real life, but the fireworks in the sky did start exploding at the precise second when the new year began. All those present on the balcony offered copious New Year greetings and raised a glass to 2005.
I watched the fireworkdisplay both live in the sky and not-quite-live on digital television. It was hard to tell precisely how long a delay there was between the live and the transmitted fireworks, partly because it was impossible to watch both displays simultaneously but mostly because it was very good champagne. It appeared to me that BBC1's pictures of the fireworks were again running approximately one second behind the actual aerial pyrotechnics. The fireworks were much prettier on the television than in real life, for which I blame the rooftops.
The firework display lasted exactly ten minutes, finishing at 00:10 precisely. The noise from the fireworks, however, continued for a further 8 seconds. With sound waves travelling roughly 1km every 3 seconds, this suggested to me that the balcony on which I was standing lay just over 2½km from the London Eye. On returning home, four hiccuping attacks later, I was pleased to discover on a map of Central London that this measurement had indeed been accurate.
Moral of the story: Don't sip your champagne too quickly or the bubbles get stuck in your throat.