Performa 30 hot water failure?

This boiler turns off the hot water supply, giving a hot water sensor fail indication. This happens when the heating period selector is off. The fault goes away when the clock turns the heating on.
My heating bloke told me it was low water pressure, and turned it up to 2.5 bar. No soap!
he will be back when his workload eases,so I would like an opinion before he arrives.
Potterton are a load of pirates, I can talk to noone there.