Hello Arctic - many thanks for your quick reply.
The new mount came from DMGRS (part number KKB101914) and I believe it is supposed to be correct for the Rover 75.
Overall it is 14mm longer - 4mm of that is on the threaded part, 10mm is the rubber body.
So - it does sound as though your explanation is correct and it is just that the body of my original mount has compacted.
I am going to go ahead on that assumption - fingers crossed.
Thank you so much once again,
Mark