Folks,
Looks like it was the front calipers, especially the front near side. The problem remained intermittent.
Calipers stripped down, cleaned, greased etc., and all seems ok now, but will keep fingers crossed in the short term.
I purchased a new MC anyway as I'm sure a suitable one would get harder to source in the future.
If, like me, you prefer to buy these things from a human being rather than on-line, for those of you in East Sussex I recommend Autolec Motor Factors. They have been able to supply a hard to get part for the C3 before and prices seem fair.
