Hi - many thanks for taking the trouble to advise me on this - I'm really grateful.
To answer your questions: Going from the back upper left of my mouth, the two back teeth of the bridge are anchors, then there is a gap and then there is another anchor tooth. Unfortunately, I'm not absolutely sure why the bridge was made with 3 anchor teeth, but, from memory, it was because the dentist who put it in (some years ago) thought that the anchor tooth that is now split (no.2 from the back, next to the gap) was weak and so went for a 'belt & braces' solution. As far as I'm aware, I don't have periodontal disease and I don't think there is any bone loss (although my present dentist said that if I just leave the bridge as it is, because its infected (but draining straight out) there will be bone loss down the road). I think the anchor tooth has been root canaled.
Again, I'm really grateful for any advice/insight you can give on the proposed procedure which I understand will take about 45 mins. I suppose also (from ignorance!), I'm not sure how it is possible to extract the bottom half of the tooth while leaving the bridge in place. Many thanks, David