At the moment I have configured the heating system to only use the heat pump, whilst isolating the two 3kW immersion heaters. This drives a 2.5kW bathroom radiator and only one 1.8kW living room radiator at ~35C and warm domestic water at ~40C. I run this arrangement on the 'off peak' electricity for 5 hours at a cost of 70p. All this is with an outside temperature of 14C, so it will be interesting to see what happens with the new tariffs when it is freezing outside.
I have also switched off the large freezer, reducing the total daily cost by 68p.