Andy, this is (one) of the standard US sizes. They are available over there from any trailer supplier and can be bought from a few US car spares places in the UK. Try USAutomotive (Bedford) who have always given me great service, or try Drawtite.com in the states. Remember that all these are not of the conventional UK two bolt fixings so will need a different adaptor plate to fit a European style towbar.
Facebook
Twitter