I have a mature ceanothus bush (about 4ft high, similar spread) We've just had another wonderful display, but now the leaves have turned yellow. It's not done this before (well in the 3yrs we've been here)
I googled it, but there seems to be conflicting advice.

It's not one that you can prune into old wood as I've tried with a few of the brances at the back, but I don't know much else about it as it was here when we moved in.

Can anyone advise please?

Bad news, I'm afraid - they are notoriously short-lived, and the wet summers the last two years won't have helped. It sounds like you've seen the best of it. :(

can you replant another in the same spot? I know lots of plants don't like it

I've never tried, but I don't see why it should be a problem - it seems the plant is not killed by a virus, but simply reaches the end of its natural life, so the soil should be fine. Good luck!

Thank you :D