Belt is about £25 from Volvo, so you’ll be able to find a cheaper one for less. Access is from under front wheel arch, you need to take liner out. You’ll need to partially take main aux belt off to fit new belt, if just checking the compressor by hand you will be able to turn it without.
There’s a guide here.
https://www.volvoforums.org.uk/showthread.php?t=208501