well ive always used my local kwik fit for air con servicing and they are fine,any dummie can connect a machine to a hose and press a button.
 
I had mine done at kwik fit last year, was a special offer £30, still working fine now.

Although I think by looking at the guy who did it if I hadnt waited whilst he did it he probably would have been scewing my car round the block.
 
Getting my air con serviced next week along with a 30k service at a local Seat Specialist (ex Seat dealer). They quoted £50-60 over the phone for the air con service. They recommend it be done every 2 years, but then, it's only a recommendation - it's not mandatory, or part of the service schedule...