It's difficult to get a picture of your proposed setup from your description. Could you post a diagram and/or some photos?

Thanks for replying.  Here is a photo of the lightwell.  It runs into the neighbours so there is no “side” to it as you can see in photo.  The otherside (behind the photographer there is a wall enclosing the lightwell the same height as the one in the photo 1.1m.

If it's raised up so the fan is blowing across the "shelf" it should be fine. 2.4m to the wall should be plenty of clearance. 

Thank you! 

Out of interest, taking advantage, in case you know, do you have any thoughts on the type of ASHP that might fit? We probably need 18kW and we have room inside to have a split pump to make unit outside smaller if needed.

I can't see why a monobloc wouldn't fit. We have a Mitsubishi Ecodan and I think that would fit in the space.

But you might prefer a split unit. There are advantages and disadvantages to both choices.

Have you had a heat loss calculation done?

18 kW is big ! How big is the house ?

@juliamc  the house is a four bed victorian semi.   A company that does retrofits says 18kw is what was used in a similar but slightly larger house that was fully retrofitted.   So I guess not too sure what is needed for us but shouldn't be too different.   Perhaps we need more calculations, the heat loss calculation @M.isterW mentioned.  

This Heat Geek link includes their cheat sheet for getting a ball-park figure for heat loss: https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/

 @juliamc  Great thanks yes looking at this looks more like a 16kw or even 14kw depending on how much internal wall insulation we can get in.

One problem is that we can’t have the heat pump above the “parapet” so it needs to be horizontal rather than vertical (which it appears most are)

Keep us updated on this installation, @moggins - it will be great to get some first hand experience documented here about what type of pump you get, how it fits and how it performs!

I’ll add your heat pump badge in anticipation of the install. 

Bear in mind the silly 1 metre minimum distance from property boundary rule or it would require planning permission. Would this apply in this case?

Thanks @nealmurphy,  we are fine and will be 1+ metre away from all boundaries so don’t need planning permission.