The OP has not said what make and model of combi he has. There are 3 or
maybe 4 different ways the boiler might detect the water flow.

I'll check later and repost - it's not my boiler.

If the CH is working then the fault most likely rests with the flow
detector (or PCB just maybe). The diverter valve is unlikely to alter
whether the gas lights when a tap goes on.

There is a bank of lights on the combi itself showing pilot light, burner on
etc. What sometimes happens when the tap turns on is that the lights all
light up to the top (top is ignititoin) I think but then fall back down to
just below "burner running" light.

If that helps :-)

Cheers, Rob.