ASHP with oil backup

[Hughg]

I’ve inherited a Daikan ASHP with the legacy oil boiler as a backup to automatically kick in when the water temperature drops to a certain level. 
 

The previous owners just said don’t touch anything and it will be fine. But after an enormous electricity bill I’ve discovered that the oil boiler backup was set to kick in below the ASHP’s own internal electric back up(35C) - i.e. never!

 

Does anyone know what water temperate to set to trigger the oil boiler? At the moment I’ve set it to 15mins after falling below 38C to try and avoid it cycling on and off quickly. 
 

thanks in advance for any advice/thoughts.