Right guys an update, this morning I woke up to no hot water and on further inspection the pump was running but the boiler was not lighting! So after applying the text book fix of turning it off and on again to no avail, I gave the side of the boiler a whack and it sprung into life! Good job the engineer was already booked!
Anyway, it was a different guy this time and he quite quickly found that the gas valve was the wrong one for the boiler, and there was a spiders web partially obstructing the flue!
So he retested it, all levels now correct and boiler firing up, and he removed the "at risk" sign from the boiler.
I was at work and so it was the Mrs who was here again, but she said that the guy wasn't happy about the fact this hadn't been found by the last guy, but none of this was on the report, so I'm guessing he doesn't want to get the other guy in trouble!