Some good news at last.
With the correct 19mm socket (not the multi-tooth kind!) I used a soft-faced mallet to beat it onto the damaged nut.
Using the new wheel-brace arm recommended by Russ and a lot of weight I have managed to remove the wheel nut without any further aggravation.
I am unsure if the gas torch I applied on a previous day tto get the other socket off the nut helped and, if I'm honest, I don't care now it is off!
Many thanks to all of you for advice, guidance and all those "Positive waves" that came my way!
Once I've replaced the front discs and got the car moving again I will be tackling my heatre woes! What a full life it is to be a Jaguar owner!