Thank you everyone for responding.
janesaysl If their mortgage company makes it a condition then I might consider that, but AFAIK they haven't done.
Bovril It's good to know that your sale went ahead without one. I don't know exactly when the extension was done, I just think (from the decor and style of the kitchen and what I recall the previous owner saying) that it was late 70s or early 80s.
AbbeyRoad I thought the indemnity insurance was a one-off and so your (future) buyers would be covered by the one you already have.
PolterGoose It's always been at the buyer's cost? Interesting.
Spickle I have supplied the certificates (FENSA, ELECSA) for all the work done since I have owned the house and accept that if I had lost them, I would have to pay to replace them, because any work done these days has to have building regs approval or equivalent certification. I just don't think it's reasonable that someone now expects me to pay to insure work done by the previous owner under a different set of legislation / standards. The other side's solicitors asked if I have building regs certificate and, when I said no, that's when they have asked for the indemnity. The purchase will only fall through if the buyer doesn't get their finger out and get on with it.
Delphinium Not enforceable after a year has expired - what does this mean? I think that at least 30 years has expired since the extension was built! If it were unsafe, I think someone (including the buyer's surveyor and indeed my own surveyor when I bought it) might have noticed by now.
Since posting this thread, I've seen that the British Insurance Brokers Association recommends that, in this situation, the buyer should pay because they are the only ones to benefit from it, but they accept that a lot of sellers get forced into it because they're scared of losing the sale.
In my case, two similar houses in my road have gone on sale since mine, and both sold (STC) extremely quickly, for more than I accepted on mine. (Well, I don't know the actual agreed price but I know the asking prices and they wouldn't have been taken off the market so quickly if they didn't achieve somewhere near it.) My estate agent has confirmed that he could find me another buyer immediately if the house were to go back on the market, quite possibly at a higher price. In view of that, I've instructed my solicitor that I won't be paying the indemnity insurance... and now I'm waiting for the response.