How can I get hot water from my Protherm 80E combined ch/hw boiler?

My PROTHERM 80E works fine on C.H but does not give hot water. [it used to but stopped]. CH Plumber turned power off and on and it worked briefly but stopped after an hour and will not give hot water. F1 shows briefly when power turned back on but then goes off and normal (temp) display shows.
CH works OK. HW display light does not come on (CH light stays on). No pilot light but boiler lights fine when CH demand on.

Is there a simple fix? Control switch/valve?

I do not have a manual