Andy,
We bought our Sombrero from "Camping Sport De-Wit" in Holland for 59 Euro, or we could have bought an Isabella Sun Shadow for 99 Euro (2m x 3m)
These are identical in design, roughly 2 metres deep by 2,4 metres wide, or 3 metres, or longer.
They either fit directly in the awning rail if you only need to keep the sun off your head, or they will clip onto the front edge of an awning - if the awning has a mini-awning cord in the front edge.
A picture of the Sombrero / Isabella Sun Shadow on our Triton is on http://www.eriba-amiga.co.uk/awning_mini.html
Or you can look at the Isabella web-site http://www.isabella.net/uk/suncanopies/shadow.html - quite a nice photo story on other uses - like wind breaks or by angling downwards - as a sun blocker.
Isabella Sun Shadows are around