Worcester Bosch Heat Pump

Tops0039

I have a Worcester Bosch Greenstar heatpump Air to water 9.5kw that is 12 years old. In the morning I find that the system has switched on the immersion and both hot water and heating temps are the same in the upper 30’s. I can leave it like this for hours and both figures hardly budge. The immersion is on continuous sapping my electricity bill. When I switch the machine off it first concentrates on raising the hot water temp then the heating flow temp. I have had WB come out 3 times. They said there was no refrigerant leak but it was a faulty expansion valve. This was repaired three weeks ago. Now the temp outside is -2 the problem has manifested again. I’m loathed to call the engineers out again as it has already cost me a fortune....or maybe I have no choice? Is there something I can look at myself? 
Many thanks. 
Tim

Rodders53

Pop over to https://forums.moneysavingexpert.com/categories/lpg-heating-oil-solid-other-fuels where there's sticky threads about your style of ASHP and the experts on it reside.