To conclude (although my fight with Homeserve is only just beginning!), it turns out that I was right about the combustion chamber door seal.
I got a local engineer in, and although at first he ignored my suggestion of the seal (!), eventually I persuaded him to have a look, and sure enough, it had almost completely disintegrated. We also noticed that one of the screws on the door, was slightly mis-shapened and would not screw all the way in.
Anyway, ordered a new door seal, had it fitted, and our heating system has now been working fully for almost a week. We suspect that the Homeserve engineer didn't go into the combustion chamber door, but wonder if he saw that a screw was slightly out, tried to tighten it, causing the door seal, which was already pretty knackered, to stop sealing, therefore causing the lockout after 20 mins.
But certainly no need for a £500 powerflush. My local engineer charged me £88, which obviously I will try to get back from Homeserve, as well as stopping my Direct Debits to them.
They have had the nerve to write to me, saying that until I prove that I have had the powerflush, they will not provide any cover. And if I don't have the powerflush in the next 28 days, my policy will be cancelled.
Absolutely criminal!