If you remove the pot, you'll be able to measure its resistance OR its value will be written on it as it will be a 'standard' value. Once you have that you then have to do a little more measuring. I suspect it will be a linear value (where at half a turn its value is half, at 3/4 turn its value is 3/4) but it may be a logarithmic (or audio) taper if the values you measures are not as I've said. Once it's out, then simply google search on sites like Farnells or RS Components and it'll show up so you can order & replace. FWIW, I'd spend good money on one (def not the cheapest) as they take a lot of pounding.

The heater temp control is an NTC sensor behind a small grill (indicated by the arrow) in the Climate Control Panel Take a torch and check to see if it is clean or broken. If not clean use a can of air to blow it clean. It then responds much better. If it's broken then visit the Volvo forums where you'll find out what to replace it with. It's a fiddly but not difficult job. Alternatively, open the bonnet and at the back of the engine under the windscreen there are two black rubber hoses going through the firewall in the centre of the car. Put your heater on full heat and start the engine to heat up the coolant. You should feel one getting hot and the other noticeably cooler as heat is pulled from the coolant into the cabin. If the temp difference is barely noticeable then it's quite possible the heater matrix is partially blocked. Frankly it's a common issue. Some people (inc me) have had great success by removing the hoses to the matrix and REVERSE flushing using mains water pressure. You'll be amazed at...

Apologies to the original author, but reversal is NOT as simple as reversing the disassembly steps.

It is vital that the nut holding the cartridge in place is not over tightened. In my years of experience I have found that nipping it up just over finger tight, putting the water back on gently to test if it leaks is best. If it leaks you do it up gently in 1 hour on the clock steps till it stops. Then do it one hour more.

Seals like those used in taps (faucets) and o-rings need only be compressed by 10-20% to give 100% seal. Overtightening causes them to leak.

The vinegar talked about should be white vinegar. Vinegar is a form of acetic acid and that helps break down any possible limescale holding the cartridge in place.

BE PATIENT and wait for it to do its job.