And.......
I haz aircon.
Uncle Google directed me to Hardwick Motors in Sutton Ashfield, who supplied 0.64 kg of R134A for the princely sum of £20.
200 yards down the road it started blowing hot.
Didn't have time to faff about yesterday, except for checking the clutch was still pulling in, (which it was), checked for leaks, (nothing visible) and tried the hp pipe from compressor to condenser, which I guess should have been hot and wasn't.
Took it back today, but noticed it was actually working
after I spoke to them on the phone.
Anyway their techie came out for a look, satisfied himself all was well and explained it was probably a frozen txv valve, due to yesterdays extreme conditions - it was 34/35 degrees.
Fingers crossed that's it for this year.