No hot water, although heating working?

No hot water from Glow-Warm 38CXI, although heating working OK. Problem started few weeks ago, hot water was fluctuating look warm then cold, then getting a bit warmer. Simultaneously the heating remained on continuously even when switched off at controls. Engineer came out and replaced diverter valve (black plastic device with three spade connector). Worked ok for a week now hot water stone cold, heating working fine. So problem, no hot water!