from that description it would seem to be badly terminated or failing connection at the MCB.  It’s relatively simple to sort out, replace MCB, trim cable to good copper and determinate being careful of tightening torque etc.  essentially an easy job for a standard sparkle. 

I would suggest that you check the service schedule includes an inspection of ALL electrical terminations relating to the ASHP.  This would be the same for checks in solar, battery an EV chargers which should also be tested in a regular schedule.