WB Combi Boiler Woes

[moorfield]

Occasionally I see boiler discussions here, so here goes nothing.

We had a new combi (WB Greenstar - recommended to us by British Gas) commissioned & tested last year, since then has failed twice to deliver hot water.

The first attempt by BG to remedy was to replace the diverter valve, which seemed to work for a while. But now no hot water again and the boiler is now kettling/cutting out when it runs.

The second attempt they tried to replace the heat exchanger, which hasn't worked. The engineer has taken the thing to bits and ruled out the valve, exchanger, and thermostat. Internally it looks in good order, no sludge/limescales/blockages. He's told me he's very puzzled by this and thinks it may be some sort of "flow" problem, but wasn't specific. Fortunately it is under warranty so now I'm waiting for the manufacturer's engineer to come take a look today. And have no hot water.

I'm not sure if I'm being bullshirted now and want to be armed with some pointed questions I can ask the next guy to ensure the right thing is being looked at/has been done.

Any thoughts, thank you?

[stewamax]

Does your central heating heat up normally?
Do your hot taps run normal quantities of (cold) water or is the hot tap flow a relative trickle?