I'm far from an expert, @dorkypark, but my understanding is that openings like air bricks are only an issue if within 1m of the bottom of an R290 heat pump. As a question to everyone reading this, if you have an air brick inside that area, is it possible to get it moved further along the wall? @Dorkypark, if moving an air brick along wouldn't cause any problems to the way the building ventilates itself, is that something you might have space to do?

Looking at the list you gave, Samsung are getting a bit of a panning on this forum for their rather high-handed customer service attitude. Irrespective of the quality of their kit, I'd avoid them on that alone, personally.

I'm far from an expert, @dorkypark, but my understanding is that openings like air bricks are only an issue if within 1m of the bottom of an R290 heat pump.

The Vaillant requires 2.1m to 3.1m. See the attached picture. My air bricks seem to be spaced about 1.5m apart so I guess it would be a case of blocking them up rather than moving them. Probably easier to get an R32 heat pump!

Posted by: @dorkypark

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the first two ruled out and the Grant seemingly a bit rubbish (inefficient, noisy and ugly), that leaves the Logic Air. By the specs the Logic has very good SCOP values and is very quiet but I'm struggling to find real world opinions on it. Does anyone have it istalled or have an opinion on what it is like?

Inefficient noisy, not sure where that statement comes from? 

How efficient the heat pump is down purely to system design, many things screw with this figure - positive or negative. Especially the design temperature, zones, use of a buffer or volumiser etc. 

If it was as simple as pick the best make, the whole thing would be easy. Best to work through the design advantages or shortfalls and not get too fixated on makes or models, that's the last decision to make, not the first.

@dorkypark, welcome to the forums! To be blunt, there’s not a huge difference between most branded heat pumps themselves. What really sets them apart is the quality of their controls and, more importantly, the competence of the installer. If the system is well-designed and the installer understands the ins and outs of the heat pump they’re fitting, you’ll likely end up with a solid system regardless of the brand.

That said, I encourage homeowners to heavily consider after-sales support from the manufacturer. On that basis, I often steer clear of Samsung, as their manufacturer-side support is notoriously poor. Grant and Vaillant, on the other hand, have strong reputations for reliability and support. I can’t comment on Logic Air.

Posted by: @johnmo

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Inefficient noisy, not sure where that statement comes from? 

I guess I'm wrong about the efficiency but the sound power level of the Grant is 65 dB vs 54 dB for the Vaillant and 52 dB for the Logic. More than 10 dB makes a big difference, though who knows if they used the same measurement method.

Thanks @johnmo and @editor for the information. I'll certainly be reading up further about what makes a successful heat pump installation (beyond simply aiming for a low flow temperature...).

@dorkypark please start by vetting your installer(s) and check their reviews and see if they can give you references to recent projects they've completed. Low temperature systems are the way to go for inefficiency, but you need that system designed and installed correctly.

@editor dont you mean efficiency (not inefficiency) Mars? 🙂

We’ve just had a Vaillant installed (literally finished today) and the installer (Heatgeek elite) and Vaillant support has been excellent. So personally an established brand versus an unknown of LogicAir is an easy decision. I agree with discounting Samsung and Grant. 
But you still need an installer who knows his stuff and such a wide choice makes me question Why?

Posted by: @judith

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unknown of LogicAir

OP missed the make, Ideal from the model name 'LogicAir' 

So not really unknown

Posted by: @editor

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...

To be blunt, there’s not a huge difference between most branded heat pumps themselves. What really sets them apart is the quality of their controls and, more importantly, the competence of the installer. If the system is well-designed and the installer understands the ins and outs of the heat pump they’re fitting, you’ll likely end up with a solid system regardless of the brand.

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@dorkypark, I don't think it's possible to overstate what @editor has said here.

As I'm sure you're aware, a heat pump is just a great big fridge, extracting heat from the air outside and heating water. How that water is then used is the clever bit, and tailoring it to a particular home is the key. Design and implementation, not brand, and that requires an installer who knows what he or she is doing and is prepared to be professional about it.

It's perfectly technically possible to put a factory-tuned Ferrari V8 engine into a Reliant Robin. It doesn't mean you'll end up with a good result at the end of the exercise.

@majordennisbloodnok As I understand it, a Robin Reliant is hardly the most stable vehicle even with its’ own intended engine on board! Now, a V8!? Eeeek! What about a conversion to a 3 motor E-Reliant? 🤯 Toodles.