As TTT suggests, the system might just need re-gassing. Kwik-Fit recommend getting it done every two years and the RAC website seems to concur:
"Most manufacturers recommend you service your car’s air-con every two years or so".
https://www.rac.co.uk/drive/advice/car-maintenance/air-con-regassing/
Expect to pay around £50 for a re-gas at a local garage (or a tyre-&-exhaust place), or several times that if you're foolish enough to go to a main dealer.
However if it's a part failure be prepared to hand over big money. (My friend was quoted £600 to replace an air-con component).