thank you Steve
Yes--I keep learning.!! The head was overhauled a few months ago and required some welding to improve gasket sealing.
The seat dropped after about 100 miles later, and the machine shop said that one of the causes of this could have been welding fairly close to the problem seat--that the heat slackened it off.
So now it has been completely renewed with new seats, and valvestem seals --plus skimming (again).!!
So I have this perfect head with this question mark over it being 1.1 millimetres thinner than when it was born.
Seemed like a thicker gasket might be a good insurance idea.
What would indicate that the chain tensioner was beyond it's limit?
Many thanks for your support on this.