Thanks all. The Admiral Rollaunch RL1000 would be ideal for a lightweight one just to take the boat out of the water here, sent off for a quote anyway though the freight will be a killer I expect.
Rogue Wave you have a PM
I don't want anything of even slightly dodgy quality as it will fall to bits in no time on the roads here!
Modding the existing trailer may be the best way to go, the prices of some of the components on the site Mark linked to suggest I could probably get everything needed for a couple of hundred quid and have the best of both worlds
Gotta go for a think now... as to how I would go about fitting the rollers to the existing trailer (shouldn't be too hard doing the keel rollers, the side ones may be the fiddly bit)