Simon,
FWIW, my car had AC when I bought it. It didn't work either & I called in a good friend who is an local expert in AC. We re-gassed the system, & it worked ok for a while but as predicted, it soon stopped cooling again!
The reason for this was the position of the condenser in the engine bay. I can't remember the reason why but with the AC test gear connected, shutting the rear deck caused real problems & the systme just couldn't cope with all the heat from the exhaust, engine etc.
We drained some of the fluid & the colour & condition of it indicated that it had been well and truly cooked! The solution to all this is to fit the condensor up infront of the radiator & if I decide to re-fit the AC this is how I'll do it.
We couldn't find a fill valve either & I can't quite remember how we did it, but when I stripped the system out of the car I discovered a fill valve set midway in one of the hoses, so you may have to remove the sills and/or some engine bay panneling to find it! (assuming your setup is the same as mine!)
Sorry this probably isn't much help to you, but if required I can photograph the piping as it came off the car.
Regards,