Something's wrong with my trailer towbar coupling but I can't quite figure
out what. Hard to explain in words, but the handle seems to be jammed, and
likewise the bit which should engage under the towball; this won't spring
open and close around the ball as I lower the coupling over the ball. I
can't see anything obviously wrong or broken but really haven't a clue what
I'm looking for (or what it should look like!) . I know this is a bit lame,
but any ideas?

Not knowing anything about towbars I'm tempted to just buy a replacement
coupling anyway, to err on the safe side. Are these things a standard
fitting? Mine is an SPP brand (Polish?) and is attached to the chassis by
two large bolts on what looks like 900mm or 3.5" centres... is this
standard, ie can I order any brand?

Thanks
David