Clutch has been playing up more and more recently, losing pressure when depressed for extended periods, so took decision to take apart master cylinder. Low and behold, white plastic retainer clip inside, broken and not keeping pressure on rubber seal and therefore allowing fluid to return. Not the correct technical names etc, but hopefully people understand what I mean.
Then had the challenge of finding a replacement on line - looks like a Morris Marina/Ital seal kit has what I need - should find out in a few days.
Joys of modified motors