Just to update on this one, in case it helps someone else.
Worked through Rich's comprehensive list (thanks again for this Rich) and drew a complete blank. However, whilst it was up in the air with the wheels off, I could see the pads were back slightly from the discs. I got my lad to press the brake pedal and could see that the pistons moved correctly, and pushed the pads on to the discs correctly, but when he released the pedal the pads were pulling back again, a few mm from the discs.
This rang alarm bells, as they should stay put, and could only be pulling back if there was a "vacuum" in the master-cylinder reservoir.
I took the cap off the reservoir, and got the lad to put the brake pedal to the floor and when he released it the pads stayed where they were and didn't pull back.
I've replaced the cap now, and driven the car some thirty-odd miles and the knocking is completely gone, and the pedal also feels better, as the first part of the pedal travel is not now being used up sending the rear pads back into contact with the discs.
I can only assume that as the new pads and discs (about nine months ago) have worn that the air above the brake fluid has not been replaced, causing a vacuum in the reservoir/master-cylinder, that pulls the pistons back in. I doubt it was down to anything the garage did when they changed the calipers, pads and discs for me, as they would have had to have bled it all afterwards - which would have necessitated taking the cap off to refill the fluid. So I can only assume that the cap should be vented, in order to allow air in to account for wear, but mine must be blocked or faulty in some way.
Anyway, I'll see if the problem returns as the pads wear further.....