The backplate is the standard 1098 one that I modified to fit the 1275, to clear the oil pump etc, didn't know at the time that conversion plates were available. The flywheel was also off a 1098, which was re drilled to fit the 1275 crank.
When I fitted a new clutch assembly, thrust washer etc and fitted the gearbox to the 1275 for the first time, everything locked up solid, so after stripping it down again, I noticed that the crankshaft flange on the 1275, was thicker than the one on the 1098 that I had, so I presumed that I had to machine the 1275 one to match the original!! Sounds like the wrong move!!
The pick up has been laid up and not used, so the thrust bearing shouldn't have worn, but I'll check as suggested.
It may be that I'll have to remove the engine and gearbox and start again, but hoping that there is a simpler solution.
Thanks again for all the advice..