lukewarm water

Hi Ecotec Plus 831 HW only luke warm, CH working fine. When running HW, flow pipes do not get warm. Diverter valve has been replaced (probably unnecessarily). When running HW, burner starts, temperature rises few degrees above setting then burner cuts out, temp falls then burner continues on...

I may be a granny and I am definitely now goosed and really do now wish I did not have a primatic cylinder tank but I do. The house I bought is ex council and the system had always worked well with the exception of air locks every time it needed drained for any repairs. I had a new boiler...