Do you know what it is that's caused the problems?
They were installed by a Mitsubishi approved company who were endorsed by the council.
The firm stopped trading a few years later, when the grants were less favourable, leaving us with no backup.
A plumbing company based 50 miles away, with no experience of heat pumps, took over the maintenance list.
I suspect the installation was never done properly, and getting even a basic yearly service has been a battle.
Every plumber/electrician/odd bod who looks at it finds something else wrong.
Mitsubishi would take no responsibility.
I reckon snow gets blown into the cabinets and corrodes everything.
We have had 3 master boards replaced (about £1000 a pop, but Mitsubishi did do one for free after I threatened to take them to trading standards for selling a product not fit for purpose). And repeated bills for call out charges and repairs (bodges)
Mitsubishi did a recent temporary fix which cost us several hundred pound, and the engineer (who does try to help) said the units only had a ten year life and would need replacing when they failed again.
But new units are not backward compatible with our plumbing and control systems, so the whole lot will need ripping out and replacing.
Of course, no subsidy for replacements....
Apart from the cost, the toll on our resilience has been massive.