You may be able to discover which system is employed by looking at the back of the heat pump; there may be one (or possibly two, one in each line) antifreeze valves very close to the cabinet back plate. (I haven’t a photo handy but if you search on the web, you will see what they look like). It these are fitted, then there should not be a need for glycol in the system. Without glycol, it is likely you will have an additive (Fernox or similar) in the system. Hope that helps, Regards, Toodles.

R32 is used in the HP circuit and is a gas right? The few times that I've bled the radiators it has been a light blue liquid that has come out.

@trbob Ah! Now, what you are referring to (R32) is the refrigerant gas used as part of the compressor mechanism; this is a sealed system and should not be touched. Other than if there were a fault and a leak of the gas, this R32 gas should remained sealed and untouched.

What you are concerned with is a totally separate circuit containing the liquid that is pumped around your radiators to distribute the heat created by the heat pump. As I mentioned before, antifreeze valves may be used but if you do not see these on the back of the pump then it may well be you have glycol in your system. Please be aware! I am not an expert but I do know that some glycol solutions are blue but I don’t think I would like to state that your system thus contains glycol! I feel sure that another reader with more knowledge in this field will be along to advise further. I repeat, the R32 is the refrigerant gas and is sealed into the heat pump and need not be touched when making changes to your radiator plumbing in any way. Regards, Toodles.

The market-leading additives for circulatory water in radiators are Fernox F1 or Sentinel X100.

It is often the case that the heating engineer attaches a sticky label to say which system inhibitor has been used for your home.
Look for this either stuck to a hot water cylinder, or next to the filling loop which is used to repressurise the system.

System inhibitors are available in differing concentrations and packaging, depending on how you are going to insert the chemical into the water.

Some homes have a Dosing Pot to add chemicals, whilst others will need you to force the fluid through a radiator air-release valve.
When adding fluid via a radiator you need a bottle with the appropriate screw-thread adapter.

Thank you Transparent, there is this, nothing else. 

During the renovation, a month or so after having the ASHP fitted we had another radiator fitted and associated pipes, so the whole system was drained down, they just topped it up again in the top of the tall towel heater radiator in the main bathroom and that worked, I guess that we can just do that again.

That photo is a great help.

The Fernox TF1 filter can also act as a dosing pot.
There's even a link to the instructions which describe how to do so:

It's typical of Fernox to design the filter so that it has another use!

That still doesn't tell us which inhibitor is already used in your pipework.

Posted by: @trbob

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they just topped it up again in the top of the tall towel heater radiator in the main bathroom

Do you mean that the system was re-filled with water via the radiator?

Surely not...
... that wouldn't bring it up to operating pressure.

There must be a filling loop somewhere, and a gauge to tell you when it's reached 1-2bar.

Thank you Transparent that is very helpful!

When they drained the system they collected the inhibitor in a container and then re-filled it via the highest radiator in the house. At the time I probably topped up the pressure with water from the filling loop. Yes I have a pressure gauge.