Hi Alpha man, cheers for the response.
I'm up in Cheshire..
An engineer has been out 5 times so far trying different things, new pcb, a thermistor, a microswitch (hot water flow switch?) and today the overheat and flue thermostats. The return filter has been cleared from being pretty clogged. Still has the same behaviour, failing a few times as it tries to fire, then gives up and sits with the flashing red light.
He initially recommended replacing the heat exchanger, the diverter valve, and having a powerflush done. He's not convinced that this would solve this problem though, and after checking the prices of the parts, has said it would make more sense to replace the whole boiler..