I've managed to do this. I have a Remora F6 air source heat pump (ASHP)
Quick overview.
I've removed the heater and the circle pump from the Balboa Spa pack (leaving the cleaning cycle, jets, lights, etc all working.
The circe pump, ASH, and a new water temp thermometer are now working independently of the spa pack to maintain water temp at anywhere between 8C and 40C.
No cost calculations done as yet, I'll update when I do :)