Hi Jonathan, came across this website which might prove useful. Not dealt with them personally though.
http://myersblacksmiths.co.uk/#/rivets-bolts/4565523902
Cheers,
Richard.
P.S. Actually just noticed you posted this in the Suppliers section back in 2009! Anyway they claim to do 3/4" BSW square nuts which seems to be what you're after!