Thanks for the info, I always prefer to use a smaller local business rather than a huge company with dozens of outlets.

<rant>
Theres not much choice left for stuff like that, once the big firms have driven all the little ones out of business they can charge what they like. Maplin are guilty of that - RF Potts in derby are just about the only independant component sellers in the area, unless you can persuade some telly-bodger to sell you some spare Rs & Cs.
</rant>

I'll get in touch with them and see what the crack is.
I've definately decided on trapezoid screw; its not too much more than threaded rod price wise its just the machining. But it seems as tho that might not be quite the stumbling block I thought it would be.
Kudos to Michael for saying we might be able to sort something out.