We have a Potterton Kingfisher boiler about 12 years old with motorised valves on CH and Water, no Frost Stat, boiler in kitchen with hot tank above in same cupboard. 9 radiators with rad thermostats on 7 of them (not bathroom or airing cupboard) Water stat on hot tank. No room thermostats.

The problem is extremely hot water at the sink, the tank is set to 55 deg C but seems to be about 80 deg C. BG changed the tank thermostat today to no effect. System works fine if CH is off, Boiler seems to carry on even though all rad stats are 'satisfied' and have turned 7 rads off. BG man assured me there was only a basic boiler thermostat that simply set the temp of circulating water, could only recommend a room stat to 'overide' boiler. I am sure this problem was not there last winter. Problem came to a head when very high winter quarter gas bill arrived!!
Do I accept BG advice and fit a room thermostat or tell them to send a better engineer?