Odd airflow fault Britony Combi SE?

Hello,
I have a Brittony Combi SE boiler. It works fine in central heating mode, but sometimes when asked for hot water the thing shuts down. All the lights go off momentarily ( including the two above the temperature controls), all fan flow noise stops. Then after a minute the 40 60 80 fault code flashes. This means fan running but does not activate switch according to the manual. This problem occurs normally when the heating has just come on, give the boiler a couple of minutes to warm up and it does not occur.
I'm thinking about looking at the airflowswitch first, but if this was going funny why would the thing work fine for heating? PCB perhaps?