So, a quick update.
Had the bearing plate replaced, and it didn't initially improve things, in that I was still getting flame failure after 10-15 minutes of service. The boiler man's next guess is that, whilst the boiler has been serviced, it hasn't had a full strip-down service and, consequently, is full of debris and other crap that is causing the issue. Before we go further, I have agreed to get Worcester out to do a full service as requested.
One thing that seemed strange is that, I am sure, my boiler has always heated water and the CH by having the flame on for about three minutes, off for about three minutes, and then repeating until the right temperature is reached. The boiler man is adamant that isn't normal, but I'm sure it always has. (Bearing in mind he also tried telling me that the hoses from my LPG bottles needed replacing as they were from 1992, but they are a year old and the 1992 was obviously part of the BS marking, I don't take everything he says as gospel)
Anyway, he did get it running for a while, with both temperature dials set to the maximum position and it hasn't 'flame-failed' for two days now. Maybe the various service modes etc. burnt off enough crap that I have a stay of execution, but I still plan to get the service done. Problem is, that it seems to stay on now and my water tank is getting to nuclear heat. The temperature gauge on the the boiler is showing numbers like 92, and the thermostat on the tank doesn't seem to be kicking in. I'm now using the Hot Water Advance button to stop it heating, as it is so very hot.
So, as a guess, is it possible that we have somehow cooked the thermostat on the tank, having run the boiler in 'service mode' so many times during faultfinding ? I'm guessing that a thermostat change is a good first port of call so, are they pretty much universal (e.g. normally open or normally closed) ?