Well as I said earlier I've a low power laser (that i was going to use on the mill for alignment) that I've played with. Taped it to a DTI magnetic stand and aligned it so it shone along the rail. You can tell any high spots as the line gets brighter. The spots are dust/dirt. Will tweak this tomorrow but the rail looks straight enough. The carriages certainly slide easily with no binding, I'll try 2 bolted on a piece of Ali tomorrow after machining a reference edge to line the carriages up to.