Best I've come across is 270mm and that one looks a little light weight for a rib perhaps?
Thanks again for all the suggestions, what I'm going to try is getting a reasonably priced jockey and clamp, cutting the lower section and fixing in an extension to it. If the join won't retract into the top section I'll lose some travel, but not sure that matters, there'll be a one-off lift needed at some point anyway to slide the tube fully down in the clamp. And getting a complete jockey just seems an easier and not much more expensive way of getting a tube, wheel and an arrangement whereby the wheel can turn to steer than trying to get and fix together the bits separately. It'll have to be as well as not instead of the existing jockey as the extended lower section won't fit below the trailer when it's on the towbar - so either keep it in the garage and put it on when wanted, or look at one of those pivoting clamp mounts I guess.... We'll see if it works!