Mick,
Sorry to have kept you waiting It was the rear brakes that seemed to be causing the problem, as you suspected.
My mechanic stripped them down (took off drum etc.) checked the pads and discs - found that one of the pads seemed to have set solid in it's housing and was badly worn (90%). He also noticed that the discs had quite a big 'lip' on them so, rather than replace them, he took them off and lathed down a millimetre or so off each one. He fitted new Ferodo pads and (touch wood) I haven't heard a squeak since.
PS. He did say that the discs will need replacing in a year or so, though.
Happy New Year.
Oops, nearly forgot - special thanks also to Strider as he (along with Mick) correctly identified the problem of the pads being seized in their housing/blocks.
Many thanks.
PS. Cost me £56 all in for the rear repair (including pads!)