Bax Combi 105 hot water issue?

I have recently had the expansion vessel, DHW diaphragm & pressure relief valve done on my Baxi but am still having major problems with the hot water.

When turning on the hot tap or the shower (run off the boiler) the boiler fires up but then cuts out and the red light over the 70 lights up and all other red lights go out.

The boiler continues to fire up and cut out every 20-30seconds and if i'm lucky eventually it may stay on and supply hot water, if not i get a cold shower.

Any suggestions of what can be wrong now would be greatly appreciated as I am worried I'm going to get ripped off for being a clueless woman, thanks