I have a 'pared back' heating / water system in my 140ish sqm barn conversion of a multi-fuel burner with solar thermal water panels both fed through a thermal store and onward for domestic hot water and heating. This all works well with one gap ... when we're away for a few days at this time of year ideally we need to safely put some heat into the building, but with the sun nowhere to be seen and the fire out have no way of doing so short of unattended portable electrical heaters (which I'm not happy doing for reasons of both safety and cost).

Anyone got any alternative suggestions?

Yes, I too would go with an air/air heat pump but be sure it can be programmed; not all can.