OP here

This job was one of the most unexpectedly-difficult ones I've ever done!

The doors were of the kind made of hardboard filled with carboard "egg box" type stuff with a thin bit of wood around the edges.

The tubular latches all had to be replaced because the shafts of the new handles were a bit too big for them as described above.

I think the tubular latches had been fitted by hammering them into a hole that was too small so to get them out involved a massive amount of tugging and levering, so much so that I nearly damaged the doors and one door started to come loose at the hinges!

The one virtue of the handles was that, rather than having 4 screws to go into the very-insubstantial door, they had bolts going through and linking up a bit like those used to attach Ikea kitchen cupboards together.

The complication was that the square shaft linking the handles had two slits at either end AND, on the other surfaces, grooves into which a tiny allen key headed grub screw in the handle's neck would go - and depending how the variable depth groove happened to line up with where the grub screw was located, you'd end up with the grub screw hardly tightening up at all and so remaining sticking out under the handle!