Been to Indespension in Leeds with the various bits that i needed for my coupling, and I think i've been sold a pup.
This is the draw tube as it came out of the coupling:
The hitch goes on the round end (right hand side in picture) and the square end is secured to the coupling with an M12 bolt which does through the housing. The whole lot slides in and out and at full extension the thing stopping the whole lot pulling out is that M12 bolt.
This is the two parts taken out with the new damper he sold me next to it:
You can see that the square piece of the old set up is pretty substantial (you'd want it to be! It's pulling your boat!). The fella in the shop told me that this other damper was "the new way of doing it" and pointed out that it's pretty much the same length. The thing is... at full stretch all the pulling force is going to be on that damper, with nothing to stop it if the damper fails, from pulling straight out the end. That can't be right can it.
( I know this is upside down but it still illustrates the point!)
Thing is that the old damper is screwed into a nut which is welded to the piston like thingy so even if i can find it I'd need someone to grind off the old nut and weld a new one on?