Solved? Well, sort of!
I sprayed some water on the belt this morning from a rose sprayer. Squeak instantly disappeared, to start again half a minute later!
Anyway, that is good news and maybe I will just put up with it (for the time being) as a mechanic I spoke to suggested I should.
Thanks for all the suggestions. I will take a close look at the belt when I fit the timing modification kit. Just glad it was nothing more serious as the bank manager has been on the phone enquiring about the progress of repairing the accident damaged D90 (yes, I bought the salvage -- a snip at £650).
Derry