Hey, Steve, if you want square section aluminum to use for a rail 1" x 1" 80/20 brand t-slotted extrusion may do for you. I use it for longer than standard rails for my little Cambo; Cambo SCs use a 1" x 1" rail -- original issue is hollow and has a spring-loaded stop at each end -- and the standards go on 1" x 1" 80/20. I used a piece of 1.5" x 1.5" 80/20 t-slotted extrusion as a base for my tandem Graphic.

I have no idea who sells the stuff in the UK. I got my 1.5" x 1.5" as scrap from http://www.kkdepot.com/product.asp?key=241 , got the 1" x 1" from 80/20 via Amazon. I don't mean to steer you only towards 80/20, there are other manufacturers of t-slotted extrusion.