When you run the DHW with the CH off, does the boiler seem to get noisier
before it cuts out? This is the symptom with mine (no pilot light, and it
cuts out sooner than 5 minutes), but it is cutting out on primary water
over-temperature (hence sounds of incipient boiling) because not enough heat
is getting transferred to the DHW through the partially blocked heat
exchanger. Are you in a hard water area? In my experience, 12 years is a
pretty good life for a modern combi.

Ed will probably be along in a minute!