R290 could be a HP show stopper for me. Seems to me we need some acceptable risk mitigations in some circumstances, other than just moving the unit further away.

Both of the units I am looking at, that produce enough kW to meet heat demand, and are quiet enough to stay within PD noise limits, are R290 units. The only location that complies with noise limits is on a first floor wall between two windows- so exposed to any wind, and no risk of pooling propane at the base of the unit. But the windows aren't far apart so impossible to meet the manufacturers’ spacing requirements. What are the odds that enough of any propane leak makes its way through an open window in enough concentration to ignite? Compared with the risk of a methane leak from indoor appliances and pipework, it must be much smaller.

Putting propane detectors near the relevant windows seems to me a suitable risk mitigation, given that the inherent risk is fairly low in such a raised location. After all, we just rely on smell as the alarm for a methane leak.

This is all fairly standard risk management protocol stuff. Any views on whether an installer and manufacturer might be persuaded to agree?

Windows that are higher than the heat pump seem ok.  Drains don't seem to be a concern.  Windows and doorways within the clearance area do seem to be a concern albeit to the lay man a bit of a head scratcher as to the real danger.  I don't think you're going to get any manufacturer to agree and I would be worried about any installer that was willing to ignore the requirements (are they legal requirements??)

I don’t think it is a specific legal requirement- other than to be safe. And there is a risk.

My point is to take a more balanced view of risks. See the earlier comments about drains. A ground floor door is obviously a different risk to a first floor window (because the ingress level is where the gas might pool) but the installation guidance I have seen doesn’t seem to recognise that.

@simonwhiteley I don't know what to say: I don't think <1KG propane is really an issue given the propensity of BBQ canisters on people's patios.  Then again, I don't write the rules or whatever, all you could do I suppose is to check with the manufacturer on their position I guess.  Vaillant Arotherm installation manual has the relevant clearances for wall mounting if you haven't seen that - I think other R290 heat pumps will be similar.  I originally asked this question because I was confused about positioning and clearances given that I was seeing plenty of examples of under-window installation.  The clearance at the top is minimal so isn't an issue and according to my Heat Geek, drains aren't a problem either.