I did it, I did it, I did it!!!!!!
I tried all sorts of things... candle wax, foot on brake, long breaker bar, etc..
In the end I bought one of these:
https://www.machinemart.co.uk/p/clarke- ... ty-12-air/
Plus some high flow hose and connectors. I had 8mm dia hose before, so I bought 10mm hose and high flow connectors.
Anyway, a couple of mins with the impact driver was all it needed!
Thanks for your help - this has been the single most difficult nut I have ever had to undo.
Andrew