Once again, my aircon has gone up the spout.

In the last three months, I have changed the compressor (with new compressor seals), got a new receiver/dryer, condensor and high pressure valve.
About ten days ago, the garage that fitted the valve re-gassed the system and confirmed that there were no leaks.
Since then it has gradually been chucking out less cool air and today the output has almost been on the warm side.
I have bought a uv light from Maplins for £9.99 in the hope of finding the leak (I think they put dye in last time round) and when I get the batteries - thought I had some fresh ones at home- I'll take a good look around. My worry, though, is that the leak might be in the the area of the heater matrix. I presume that, if that is the case, it's curtains for the aircon as getting at the matrix means practically having to dismantle the car, unless anyone knows any different, that is. I've never explored down that part of an Omega!
Every time I get the sodding thing regassed it's costing me about £70 and this is the third time this summer. The money would have been better put aside to get a newer car!