I took the elevation drum apart last night, with the help of lots of magnification. Fascinating miniature engineering and all looks OK. So how can the graticle not move when the threads are (almost) directly engaged. Either the base of the lead screw is finding movement at the base of the turret, which seems to have been disturbed previously as the screws have been undone, or the lead screw nut threads are worn. The lead scew nut is split into two halves and presumably can be adjusted by shaving off a small amount from each half to tighten it around the lead screw threads.
It is almost worth what I paid for it just to see the internals. No, maybe not!.
Detent grooves picked clean of hardened 70 year old grease at 2am this morning, greased and re-assembled. Nice clicks. I will try it again today and see if I have inadvertently changed anything.