I replaced the jockey wheel today.
Had to splay the metal supporting arms out as the axel/bar was seized into the plastic wheel hub, but readily done by hand.
Then had to power saw half the plastic wheel away off to get the axel bar out. Once the rust was filed off and greased it went readily through the hole in the new wheel - see picture; had to spring the arms back to parallel, but again easily done.
So all told the wheel from Banbury Trailers was exactly the right size and basically spot on for a 1 for 1 replacement.
Also there's a picture below of the bar - it's 20mm outside diameter and 118mm long overall.
Hope this helps