Its a 19mm head .I had the same problem last year.I totally ****ed the head of the nut,made **** of it.I ended up heating a 17mm socket with my dads gas torch till it was red hot,then I fixed on an old 9" extension,and tapped it onto the nut using my lump hammer .When it cooled a minute or 2 later I had a very tight hold on the nut.
This method will work.
I cant remember if I used a 4" and a 3" aswell .Just have everything ready and worked ou before you heat the socket.