@rumballd

The same volume of water, at the same temperature, contains approximately 3488 times more heat energy than air. The actual amount in air will no doubt depend upon the moisture content.

If a heat pump is producing 12kW of heat energy at a COP of 3, then it should be absorbing 8kW from the air flowing through the Evaporator. That is quite a large volume of air. The colder the air, the greater the volume that will be required.

I for one will be interested in how you resolve the apparent problem.

@rumballd so if it was facing out when you viewed the house, that's your quickest option, it suggests the pipework will cope with that rotation. Electrical panel access is not needed very often so you might have to just accept that. So long as you can still reach the isolator in an emergency.

I looked those grant MI's,  amazed by the lack of details   If you followed them to its logical conclusion, it would be acceptable to install it in an enclosed box, provided it was large enough to meet all those clearance dimensions. There is NOTHING that gives you any more detail that you can point to as not having been met... quite unfortunate. The thing you have to fall back on is common sense and physics. Neither of which gives you a lever to force your installer to sort it.

here's the diagrams from a samsung which might help. ok, so its not the same unit - but the laws of physics don't change from one vendor to another. If they have intake and exhaust in the same places, how you need to site it won't change much either. the samsung MI is very clear about what to do if enclosed on 3 sides (middle diagram in the left hand column). So you're on the right track to turn it to face out - and/or DIY an air baffle.

It's a pi$$ poor location for the ASHP, there will be an efficiency penalty. The Grant installation manual is a copy of Chofu's Japanese detail. The 'clearances' seem more maintenance and access clearance requirements, not minimum airflow clearances. Japanese installers of ASHPs will be much better educated and knowledgeable than the guys who installed your ASHP, and they probably would not do daft things like bang an ASHP in an 3 sided alcove with 600mm clear to the fan outlet. Fan outlet should be unobstructed as the the velocity throw will be several m/s. The inlet clearance is a little less critical, and can be reduced down to 100mm for wall mounting provided the fan outlet cannot recirculate.

ASHPs harvest enthalpy from the outside air, the warmer the air, the higher the enthalpy. Anything that mixes cooler outlet air with inlet air will lower the mixed air enthalpy going into the ASHP. The ASHP will handle that, but just work harder with lower CoP and end up cycling into defrost more often in low outside air temperatures.

A south facing location is always best, & direct sun or solar gain on the unit and the wall behind the unit helps. Brick, block and stone walls have a high thermal mass, absorbing heat during the day and releasing it overnight. Free and unobstructed air to the ASHP discharge and a location well clear of fugitive, acidic or corrosive emissions such as nearby boiler flue outlets.

All common sense really, but that often gets sacrificed on the altar of installer convenience.