Central heating system was working fine. Ran out of heating oil, (so much for the top-up service) and had to have the boiler flushed and re-started. Engineer re-started the boiler and left. Soonafter we realised, we only had hot water and not heating. Had engineer back who fiddled with the Mid-position valve, and suggested we needed a new one. Just replaced MP valve today, but find that we can't get either heating or hot water now. Boiler won't fire up under any circumstances. Checked the wiring several times appears to all be connected correctly. Any ideas other than the new MP valve being faulty?
Is there power going to the burner and does it make any attempt to fire?. If there is no power at the burner then the electrics will have to be checked. It is possible the burner is controlled via the 3 port valve auxillary switches.