An update on my situation. I determined that the problem wasn't the clutch air gap after (1) measuring with a feeler gauge to check the gap (from the top I should add) and (2) starting the a/c and noticing that the clutch would engage briefly and then switch off (low pressure cut off). Obviously (2) is the easier check and would have put me on the right track sooner but I got there in the end.
So, after determining that the clutch was OK I took it in to have the system evacuated and recharged. Voila, beautiful cold air. The cost was $195 for refrigerant and labour. They also added some dye in case there's a leak (fingers crossed).
The other thing that might be of interest here was the low side pressure was under 40psi and high side over 200psi with a vent temperature of 55 or something. Quoting these from memory right now cause I left the service docket at home but I can check these if anyone wants more detail.
Bottom line is I'm a happy camper right now. Just hope I don't have a leak.
