I have the same age car as yours, and the same problem last year. Turned out that I had a micro hole in one of the pipes and had leaked all of the gas out.
I took it to a guy in Maidenhead (Vanchiller) on the Cordwallis trading estate who were absolutely brilliant, spent the good part of a day on the problem, had to order the part, which I picked up from SGT in Taplow, and charged me £165 for re-charge and 6hrs of labour.
As it happens, I have to take it back in this year, 'cos it's gone again......bummer!!
Don't take it to a local dealer, or the SGT cowboys, 'cos the Vanchiller guy told me that they just come to him to fix it anyway, so that'll save you some dollar! I'll get the phone No. for you and post it tonight if you want(I'm on nights at the mo, so won't be logging on 'till later......hence the time of this reply!)
Where in Cookham do you live mate? Where do you go drinking??
Drive it like you nicked it!!!