It is usual to prepare the groundwork and use solid blocks or slabs on top with gravel in between the blocks for drainage purposes and of course it must be levelled of adjustable feet employed for levelling on the  base of the pump.

The sensor is often sited on the rear of the pump case and should sense the ambient temperature at the rear of the unit. When the pump is running, the airflow will of course be the exact temperature of the air sucked into the pump. There are external sensors available for some makes / models of pumps which are normally sited on a northernly facing wall thus not influenced greatly by sunshine. Interested to hear you are having a Homely smart controller - I am very pleased with our Homely controlled Daikin ASHP. Regards, Toodles.

@toodles thanks that makes sense regarding the temp sensing, and confirms what I thought regarding the base. The fact that he mentioned it on day 1 as well- clearly part of the initial plan but now they are over running with the job I just need to keep an eye on them to make sure they don’t cut corners.

Good to hear you are pleased with the homely controller as well

Hi @benson - the outdoor temperature sensor is often within a piece of plastic tubing which hangs at the back of the ASHP.
Somewhere on the forum there's a photo of one, but I can't find it at the moment.

2: The external unit will drip condensate onto the ground.
So your gravel bed needs to allow that to run to the side and down into soil.

If the gravel layer is too thin or the soil unable to absorb the water, then your path will turn into an ice rink in winter.
You know your soil better than the installer.
Use common sense rather than any formula!

3: Heat pumps are often supported by heavy rubber 'feet'.
Here's one as sold by BES

Depending on where you live, this may be all that's required.
However, there are places where heat-pumps get stolen by 'gangs' who come equipped with a trolley jack. 😥 

The ASHP itself will invariably have bolt-holes by which it can be secured...
... in which case you might want a concrete pad to bolt it onto, rather than rubber feet.

There are a number of types of 'security bolt/nut' which can be used, rather than a hex-head which could be removed with a spanner.
That means a thief would have to use an angle-grinder on the bolt-heads... which is noisy and attracts attention

Some installers might suggest fitting a couple of strong eye-bolts into the wall behind the heat-pump before it's in position.
A hefty chain can then be used to secure the unit to those eyes.

No mechanism can be completely fool-proof.
Given enough time, a gang could use bolt-cutters, angle-grinders and sledge-hammers to bag themselves a nice new £5k unit sitting outside your house.
The idea is to make your heat-pump less attractive than all the others in the neighbourhood!

@benson it’s unusual not to have a solid base under the heat pump. Typically, a concrete base or concrete slabs are used, so I’d definitely question the lack of one.

Similar to what @transparent mentioned, the Zeus Articulated Mounted Feet have been widely accepted in the industry for providing stability:

RENEWABLEHEATINGHUB.CO.UK "https://renewableheatinghub.co.uk/straight-steady-why-your-heat-pump-should-be-level"

Straight & Steady: Why Your Heat Pump Should Be Level | Renewable Heating Hub

Level heat pump installation is vital for efficiency, compressor protection, and neat aesthetics.

Thanks all. It will be mounted on anti vibration feet- that I know. Thus, I have emailed them and asked them to ensure that this is done properly. I cannot see how anti vibration feet on their own are sufficient for a heavy unit, on sloped gravel, and that article above is really handy in terms of knowing now why it needs to be level.

First the ASHP has to sit sit level to with in the limit prescribed in the manufacturers instructions.

The important bit about the weather compensation is the probe can never see the sun. This screws up the temp registered by the heat pump. Being on the sunny side of the building makes little or no difference as the unit takes energy from air which is the same temperature where you site the unit, due to the vast quantities of air consumed.

Just make sure the morning or evening sun cannot see the temperature probe on the rear of the unit.

@johnmo thanks I will be sure to check that. It is a relatively shaded area even though it is south facing so I think we should be ok.

To update on this, it transpired that the installers intended on using brackets to wall mount it. To be honest I’ve never seen a wall mounted ashp from scouring forums such as this, photos of installs etc and not least the companies own Facebook page which shows them all with a nice tidy base…

when I reminded them that the quote included ant vibration feet and thus assumed it would be on the ground, they said it could still be wall mounted with the anti vibration feet. Having seen the design of wall brackets I have no idea how this would work….

Long and short of it is Ive told them to prepare a concrete base and install it on the ground. There’s absolutely no practical reason to wall mount it other than save their time. 

literally have to watch these people like a hawk but no different to 90% of contractors we’ve had over the years in fairness. 

Wall mounted Aircon units are very common.  Many ashps include instructions for wall mounting.  Perfectly legit as long as the wall is strong enough and you have sufficient clearance.  However perhaps more likely to transmit vibration to house so ground mount preferable imho.

@benson I think that ground mount is the better option; there is always the risk of vibration being transmitted into the building structure from the pump - so a good choice there! Regards, Toodles.

@benson I’ve come across ASHPs mounted on walls before. Our neighbour has a large unit attached to the side of their house, but I’ve only seen this done when there’s limited ground space. From the photo you shared, it looks like you’ve got plenty of room, so I think you’ve made the right choice putting it on the ground. This will help minimise any risk of vibrations or reverberations being heard inside the house.