Thanks emoose, I was looking at the possibility of replacing the pipes if Autokool couldn't help, but I nipped in last night as they were getting ready to close and explained the problem. The valve is an outdated part that they don't stock, but one of the guys thought he might just have one in his 'bit box'. Sure enough - a brand new high pressure service port valve - direct replacement!
Brilliant service and very knowledgeable, and only a couple of quid!
(Later version threads apparently are different/smaller.)
ATS have been brilliant - I paid £40 for the full regas in August, was blowing nice and cold, but by the time I'd driven the 2 miles home and got out, I could hear hissing and sure enough, the top corner drivers side of the condenser was expelling pressure and oil . . .
Replaced the condenser, took it back and they refilled it - no charge - it worked fine for 3 or 4 weeks than gradually was getting less cold, took it back and the guy went all over it with the UV torch and couldn't see a leak, he made it a kind of personal challenge and was really determined to find it. Eventually wondered if it could be the valve, so charged it up with nitrogen and dye, told me to clean the ports up and run it for a couple of days, then bring it back. Again - no charge.
Sure enough the valve and the inside of the cap showed up the leak.
Now just got to take it back for another regas . . .
@N5Fav - most of the gas had leaked out and it was only nitrogen anyway. The cores aren't replacable, the spring and valve pin are held in by a 3 armed spring steel clip that has hooks on the inner end that latch into a shoulder on the inside of the body. (see photos)
Thanks all for the input.
Alastair