Saunier Duval Boiler - SD30e cutting out?

Hi, I have an SD30e combi boiler (exactly the same as a Glow-worm compact) and recently a diaphragm had to be replaced on the diverter valve, which is a common problem.

The hot water wasn't working for about 3 weeks before we could get someone out to replace the diaphragm. During this time the heating also began cutting out on demand and the light illuminating. Everything fires up fine but the heating will cut out after anything between an hour and four hours time. I reset it and the whole process begins again.

This problem is still happening, does anyone know what might be the cause?
Apparently there's another valve and diaphragm as a safety if the pump isn't working, could it be that?
It seems odd that this problem only began some time after the hot water stopped working.

Any help appreciated!!!