OK, time for another update, as I saw a consultant again today.

My GP had also asked the opinion of a cardiologist friend of his, who had
said that five years ago he would have recommended an artificial valve
for a patient with my profile, but now would say go for a tissue valve.
Not only are they getting much better, but he thought that advances in
techniques would mean that, by the time it needed to be replaced, heart
valve replacement could be a day surgery operation. He also pointed out
that stem cell research is promising a total revolution in body parts
replacement, which could easily happen with the same timescale.

This is a diy group - we'll let you off the angle grinder but that's the
limit.

(Good Luck, Colin!)