Worcester 350 rsf fault?

My Worcester 350 rsf boiler works intermittently. The heating will work until I use hot water, then when I turn the hot water tap off the boiler fan stops and the burner goes out, but you can still hear the pump running. Its not the fan assembly as I’ve had it tested by someone competent and whom I trust. After waiting with the heating selected as off for some time the combi will work with the same problem. Any Ideas out there?????????.