Merry Xmas everyone! Success, though I can't tell you exactly why! I stripped the transport and cleaned and lubed where appropriate and cleaned all board edge contacts - seemed worthwhile before investigating chips etc - and lo and behold, the problem has gone!
Moral of the story for me: if a well maintained and restored machine suddenly develops a fault, do some servicing on it before suspecting anything more complicated!
Wishing everyone a Happy New Year in advance,
Chas