Midget -
here in the States, I found and use 1/2 inch diameter (12.7mm) carbon fiber straight wall tube. You can get a 60 inch (1524mm) long piece for around $18.00 - or you can look at Kite Stores and they may have the same but 48 inches (1219.2 mm) long for around $10 each.

The long one also works fine for a mast - or use two short tubes with a ferrule/joiner in length. My first boat (not a great performer but scaled after my big cat) used aluminum tube. Kind of heavy compared to the CF tubes. If $$$$ too much, look at a fiberglass tube. A bit more weight, but less cost.

Probably a high shipping cost, but here is an idea of sizes and costs. Note they also sell heavy weight rip-stop nylon or polyester fabric by the yard for sails if you decide to make your own. I use the 1.4 oz. stuff. A bit stiffer than the lightweight 3/4 oz. spinnaker cloth.

[link=http://www.kitebuilder.com/catalog/index.php/cPath/33?osCsid=6b5a68fc148978c6f7468a36f5c06e3a]The Kite Shop[/link]

You should have similar shops there in the UK for the kite-builder guys. Lots of cool building stuff at pretty good prices.

Dick