As Fargo said, I wouldnt concern myself about the 12 degree difference - this is just technical talk , most heating engineers are only concerned that the rads heat up.
This is your hot water cylinder, it will have an immersion heater which is turned on/off by an electrical switch, The other control on this is the hot water cylinder stat, which controls the temp of the hot water. very important that this is kept over 65 degrees to prevent legionella building up in your cylinder.
Did altering the pump speed as suggested by Harvard make any difference ?
Do you thermostatic radiator valves (trv's) on the side of the radiators ?
cheers
ian