I've had a look on ebay and there seem to be several types:
1. Stainless like the ones Channel Ribs has. Look really good. Some seem to have cranked arms as well - so as to place the wheels forward for better balance I guess.
2. Aluminium ones like the above with pheumatic tyres.
3. Nasty looking plastic ones that appear to have to be mounted on the centreline of the boat somehow.
I think either 1. or 2. will be what I need.
I guess the boys need a pocket money raise