Right everyone. I took the advice and went to see the Seat Post Man and I'm bloody glad I did. What are the odds on BOTH my recent requests regarding bike problems I had being satisfied by people living in Chorley, 18 miles from my front door? Either or both of them could have lived from Out Stack on Shetland to Lizard Point but no, BOTH of them live in Chorley! I'm amazed by that.
Anyway, while I was using my legs for climbing in Wales, John, the Seat Post Man was fettling my seat post and a good job he did too. Actually faster than I needed it so I'm well pleased with that. I'd strongly recommend him to anyone with a similar problem because he's done an estimated 10,000 of these and reckons anyone with well-meaning advice should be roundly ignored on the basis they've done maybe one or two in their time and got lucky or possibly watched a youtube video and know precisely nothing about the subject. And on seeing the evidence, I strongly agree with him.