Hey there @VenerableBede :)

There’s definitely experts on here such as @hydrosam who can try to advise.

Could you show us some photos of the current setup so that we’ve got something to work with? If you could do that while you wait for other members to stop by, that would be awesome.

Any problems, give us a shout!

Is the heating entirely piped in 15mm or do you have 22mm (or 28mm) for the main pipe runs and 15mm branching off to the rads?

Unfortunately, as I am only considering buying the house, I do not have access to take photos. I’m not tech savvy enough to know how to add photos here. The pump is out in the garden, about 12 feet from the house. Large, free-standing radiators have been bought but not fitted. Two little 15mm pipes have been inserted through the ceiling in a corner apparently to be extended and connected to the radiators. Radiators are 3-panel.

I think you’d find it difficult getting an ashp installer to take over a half finished job.

Do you know who has designed the system and done the heat loss calculations? I would be more concerned about checking that to make sure it's been specified by someone who knows what they're doing.

Most of the time when people are unhappy with heat pumps it's down to poor installation. They require more skill than a gas boiler installation so you need a good company to fit them.

Most things are possible if the system has been designed properly. I have 22mm feed with 15mm drops to radiators.

As others have said, it would be beneficial to find out who has done the design to make sure what is partly there is up to the job. Also, if getting a mortgage, check lender is happy with the current set up. 

Thank you for your advice. I am not sure about the half-finished job as the person doing the work died. I cannot complete the job myself. I am reassured by Neal’s answer but will have the property surveyed by somebody who knows about heat pumps before committing myself.

I’d be inclined to review the whole installation given what you’ve said. For an installed system, it’s generally said that if you can achieve temperatures required with a 50 deg flow temperature, you can fit a ASHP without changing the pipe work or radiators. In your situation you simply will not know

OK

Where would I find a reputable person to check the system?

If it were me @VenerableBede I would give a few specialist installers a call and ask them to quote - from their data, they will tell you if they can use what’s there or (more likely) install completely to their own design

There is some info here https://www.gov.uk/guidance/find-a-heat-pump-installer

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