Andrew,

In my experience, Ravenheat are very helpful on
the phone, and will talk to you about it.

OK, great. Thanks.

Cynic,

Incompetent installer?

Had crossed my mind, of course, though he's done a fair bit of other work
for members of the family with no evident problems.

Chudford,

Does this happen with just the hot water or with the central heating?

Just the heating.

Does the boiler actually light fully...

From what I've seen (or rather heard) yes, it does. Then immediately cuts
out, fires again, cuts out, and so on, always with a considerable thump.
Eventually it lights and stays lit.

Thanks to all for the thoughts and suggestions.

Bert