Hello
Roy748Bip I have/ had the same problem with the master cylinder,
and I made the mistake of replacing the master cylinder without checking the bearing on the pressure plate, as a result the pushrod was spinning and scored the cylinder resulting in a useless master cylinder, so if you do go for an oversized master cylinder make sure the pressure plate bearing is running freely, or for about £6 replace it. Maybe also check the pushrod as it maybe rubbing/wearing down. Take it to a dealers and ask them to compare.
good luck mate and keep us posted.
regards
TopiToo
[Edited on 15-07-1968 by TopiToo]