I
think it's 30mm, Rob, but, as you say, easy enough to check. Also, measure the depth from the base of the nut to the end of the shaft to make sure the new socket is going to be deep enough (you can also get extra-deep sockets). Machine Mart do them but there are plenty alternatives out there on eBay/Amazon.
https://www.machinemart.co.uk/p/lase...lf-inch-drive/