Well I bit the bullet and bought this one.
LINKY
Grasshopper were the cheapest suppliers that I could find given the amount of time I was prepared to spend surfing.
The sale was a good experience with excellent communication throughout and I'd really recommend them.
I fitted it this evening and found it pretty straightforward if not a tad fiddly. It would be much easier if the rainguard was fitted to the awning case before it was attached to the wagon
beforehand, but I didn't have that luxury.
The guard is a thick shaped rubber strip, with an aluminium reinforcing strip and 3M tape alongnone edge.
It attaches with the sticky strips along the length of the awning casing but stops and starts againfor each of the two mounting brackets. This leaves gaps for the water to dribble through, I may silicone these later, I'll just see how it goes for now.
Only use and time will tell if it works and stays on.
If you are thinking of such a device and need a Q answering, just ask.