As an update, I had a rear caliper (was seized) and a disc changed and the braking issue seems to be much better. The brake lights no longer stay on that extra second after release of the pedal and they are slightly sharper. Turns out the rear brake was binding after all so was likely contributing to the problem.
The brakes do apply themselves still after maybe the third or fourth journey of the day, but only very occasionally. Although lifting the pedal up does seem to solve the issue, I don't think its the return spring as coming off and then back on the throttle solves it too. Maybe there's a vacuum issue somewhere and the stuck caliper was worsening it.
For me the car is worth keeping on the road purely for its generous space, great fuel economy (not long done 750 miles on less than a full tank and otherwise great reliability. Including purchase price, MOTs and the yearly oil change the car has cost me £2k for four years' driving.