If you're lucky, just needs gassing. Your worst case is that the compressor has seized. Best practice with a running car is to run the AC every week or so, even during the winter, to avoid seizure. Either get a professional or search YouTube for AC compressor and clutch faults. A pro will probably diagnose it in seconds.

I've never had a compressor seize and cannot recall how that shows up; I think you can see or hear the clutch struggling to turn it.

After being atood for 18 months and now back on the road, the air-conditioning on our Discovery 2 is no longer working. Is it likely it just needs a recharge or should I be looking further at replacing anything?

You could try bridging the pins on the pressure switch to see if it drags the clutch in, this would tell you if its a re-charge/ pressure issue.

Ive seen a partially seized compressor, but that was evident when you couldnt rotate the inner section of the clutch without it feeling lumpy. The ones Ive stripped are just a swashplate pump inside. They have a little electromagnetic clutch on the drive input side, so even if the outer is rotating, it might not be engaging for interlock reasons anyway, but you can see the inner not spinning visually really easy on the ones Ive dealt with.
If the gas pressure is too low from seeping past the seals, the compressor wont engage usually anyway as they have pressure based switches that inhibit the compressor from running to avoid damage.
I dont know if disco2 has obd2 era control, but if it does, the system pressures will be in the live data no doubt.
I've got a set of the dual pressure gauges and hoses with connectors and get roped in for family cars in my alternate monkier of garage of dad, but the main issue is you charge the gas by weight, not system pressure. So you need to have a way to extract the gas to a cylinder, weigh it and see how much liquid freon to add, and if you need to add pag oil etc. If I know the weight etc, sometimes I just vac the system out of everything and charge with that weight dispensed from one of the freon cans. Just so happens most of our cars take exactly one can of cheap aerosol "alaska cool" freon that you can still buy. You have to make sure you buy the right gas if buying it yourself, fortunately mine are all 10+ now so I can keep to just having R134a.
My personal recommendation, just go to a recharge place, they'll take the gas out, weigh it and charge to the manufacturers spec, and if it doesnt work then, you can start digging round in the ecu data or getting into the wiring.
You can get dye injected to uv test for leaks too, I have some of that and its been really helpful finding leaking seals etc.

Many years ago, I went on a one-day training course on AC, but I've forgotten most of what I learned. Unless you value your time very cheaply, I think you'd be better going to a pro. Probably just by looking and listening, and certainly by connecting pressure gauges on the high and low sides, he'd analyse it in a a couple of minutes. For what it's worth, I might also point out that it is now illegal for an unqualified person to work with refrigerant gases

I might also point out that it is now illegal for an unqualified person to work with refrigerant gases

Can you give me a link to the regulations where that is stated that its illegal to diy regas a car airconditioning system at home?
Because searching round the net, there are hundreds of guides from reputable big brand companies like halfords, the RAC, and others happily walking through the steps needed.
Also, there's no limitations on anyone but specialists offering professional service in the legislation I found on the subject here :

Many years ago, I went on a one-day training course on AC, but I've forgotten most of what I learned. Unless you value your time very cheaply, I think you'd be better going to a pro. Probably just by looking and listening, and certainly by connecting pressure gauges on the high and low sides, he'd analyse it in a a couple of minutes. For what it's worth, I might also point out that it is now illegal for an unqualified person to work with refrigerant gases

Not according to Halfrauds, just bought a bottle to top my van up myself.

"It is against the law to work with F gas if you do not have the correct qualifications. You could receive a civil penalty for breaking the law. See the Environment Agency enforcement sanctions policy.

If you see someone working with F gas that doesn’t have the right qualifications, report it to [email protected]."

Source https://www.gov.uk/guidance/qualifications-required-to-work-on-equipment-containing-f-gas

​

Stationary refrigeration, air conditioning and heat pump (RACHP) systems qualifications​

Is the section that mentions the hard requirements for ie household mini split systems that do not come precharged with zero loss connectors, and reefers on trailers and vehicles >3.5t.
For those it mentions you need the certifications to work on that class of vehicle or fixed installation.

Further down is the relevant to vehicles under 3.5t requirements section:

Mobile air conditioning systems qualifications​

You must have a qualification to recover refrigerant from mobile air conditioning systems in:

• goods vehicles weighing less than 3.5 tonnes
• passenger vehicles with 8 seats or fewer

Ie a scrapyard dismantling a vehicle aircon system, to stop them snipping the pipework and releasing it to the atmosphere. None of the other requirements in the >3.5t section are listed.

Lets not muddy the waters with misquoting things and doing the governments job of scaring us into not topping off our own car aircon here.

Once upon a time I also used to work with white goods and we had a unit to recover refridgerant from scrap fridge and freezer units, but the owner of the business was so greedy he used to snip all the copper lines after it was unloaded after collection before we got there in the morning to process it because it was quicker than degassing the units properly. And local authorities paid this company to dispose and degass these units responsibly.
That is the class of person the regulations are put in place to stop...

@jmp49 I'm not being argumentative, genuine question. Just above the statement I quoted is this
"

F gas for installation, servicing, maintenance or repair of certain equipment​

The following rules apply if you plan to sell F gas to a customer who will use it to install, service, maintain or repair:

• stationary refrigeration equipment
• stationary air-conditioning equipment
• stationary heat pumps
• stationary fire protection equipment
• refrigeration units of refrigerated trucks and trailers
• mobile air conditioning equipment in vehicles weighing less than 3,500 kilograms with fewer than 8 seats

You must check that your customer and their employees hold:

• company certification to work with F gas
• qualifications to work with F gas

A person without qualifications can collect, transport or deliver F gases.

You must keep records of:

• the customer’s certificate number
• quantities of F gases they buy"

This seems to say that certification is required for sub 3.5t and less than 8 seats. Am I missing reading it or is there another section that gives exemption?

I used to have my F-gas for split domestic and commercial systems

Anyone with a bit of common sense could use the DIY kits on cars, as I have many, many times

Luckily I only have one neighbour and I fixed his last week so unlikely anyone is going to grass me up

the diy kits dont contain f gases. they are a 'substitute' gas. no idea what it is but it doesnt cool nearly as well.

If you see someone working with F gas that doesn’t have the right qualifications, report it to [email protected]."

Who wants a Bamboo* label applied to them, I don't.

*Bamboo, biggest grass in the world!

Just had ours done, £30 for the diagnostic/pressure test and then £66 for the regas. Good as new.

I like to do stuff myself and save money but for that money for somebody else to do it’s not even worth me lifting the bonnet.