This is only a guess, i have not seen how the parking brake system works on those.

It sounds like one or more of the pistons is retracting during several hours parked up, that isn't likely on the front brakes which really have no way of retracting assuming they are normal sliding single or opposed twin piston calipers.

My guess is that the rear calipers have ratchet type adjusters inside for the parking brake, i suspect you have to wind them back in to fit new shoes, i wonder if the adjuster isn't holding in place, has seized maybe.

If that is the problem, i wonder if someone has adjusted the parking brake cable up too much to compensate for this so now the adjuster can't ratchet anyway....as i say this is only a theory but might be worth looking at.

There has been no work carried out on any part of the braking system, apart from two yearly change of fluid.

No (what should be) normal brake servicing?

Update:

The fault has been traced to a defective rear caliper.