Can anybody recommend a car Air/Con specialist in the Oxfordshire/Buckinghamshire area?

Had my air con recharged at ATS using a £33 Amazon local voucher after buying my car a month or two earlier and found the A/C wasn't very good. Took about and hour and a quarter. They said all was fine, no leaks etc (The deal specifically states a leak test will be done before attempting to fill and the fill will not take place if a leak is detected). Drove a few yards down the road and it was obvious it still wasn't working. Took it back and they said it sometimes took two goes to get it right and asked me to come back next night. Same thing next night. This time they told me it wasn't working before I left. They said there might be a leak after all but their equipment didn't detect it and suggested I see a specialist A/C person. The ATS system seems to be more or less fully automated. To be fair they did try twice and refunded my voucher code when they couldn't get it to work. Can anybody recommend a car A/C specialist in the Oxfordshire. Buckinghamshire area?