Ok well, this is what has happened so far:
Boiler "covered" by Homeserve.
1. Boiler has trouble kicking in with hot water and starts clicking on and off when it does get going, I call Homeserve and they send an engineer.
2. Engineer changes the water switch (is that right) so it now starts ok. He leaves.
3. I run the hot water and it starts immediately but now clicks on and off is the best way to describe it. I call Homeserve again.
4. A different engineer comes round and changes the air switch at the top of the boiler. It works ok and he leaves.
5. Next day same problem. Clicks on and off but this time with heating as well as hot waler. I call Homeserve AGAIN, having done a bit of research, and ask them to make sure they have a combustion chamber seal with them.
6. New engineer turns up, agrees with me that it's the combustion chamber seal, but doesn't have one in his van.
7. I am still waiting for them to get back to me, so I thought I'd do it myself. This clicking on and off is driving me mad and it can't be any good for the boiler.
How come TWO engineers didn't spot a perished combustion chamber seal? It's a £5 spare part and, from what I read, frequently perishable.
Oh - he told me that it had to be a special seal so as to be fireproof.
Well I cut a tiny fingernail sized bit of the existing, perished seal off, spiked it on a pin, and tested it with a lighter. It burned with a thick black smoke as you'd expect any foam to burn - i.e. the seal is not fireproof. The engineer was lying.