No hot Water from Vaillant Sine 18?

Problem with Sine 18. Hot Water

CH OK.

Changed water valve diaphram (Several times over the years) which normally solved the problem.

Burner doesn't light when hot taps are on. Water pressure OK.

This time I think it may be the thermostate capsule. which has never been changed from new.

I have a brand new Diverter Valve but reluctant to change it if it's just the capsule

Any suggestions will be greatly appreciated.