Can anyone point me in the right direction with this problem.

I have a Baxi Bahama 100 Combination boiler with an annoying intermittant
fault.

Occasionally when the heating is working fine and I switch on the HW the
flame light goes out and the water runs cold.

When HW tap is then turnd off the boiler clicks a bit but the heating
doesn't fire up - I don't think there is any pump flow either

The boiler clicks when I tuen on the HW tap so it is sensing that so I think
I can rule out the flow switch.

I am thinking it could be the CH flow switch but would appreciate any help
from experts or people who have had this fault.