Heatline C24 not firing up!?

Can anyone help please...

I have a Heatline C24 boiler which is now just over 3years old. It has started playing-up and won't come on in the morning until I reset it a few times.

I've had a qualified corgi plumber working on the boiler for a few days now. To date he has replaced the PCB and also the ionisation electrode and the other two probes.

Now the boiler occasionally fires up when I switch the hot water on. But, will not fire-up for the central heating. The plumber says we need to replace the gas valve. Does that sound right?