Thanks for your replies chaps.
Your comments Martin54 reinforce my concerns. I think it safe to say the idea of using a threaded bar has been scrapped. Now to find a supplier of pulleys and belts..... I found Zapp Automation (http://www.zappautomation.co.uk) are they good? Are there any better ones available?

WRT "business use" It would only be used to prototype, I get my parts laser cut commercially, but need to do so in quantities of 100+. When first creating a new product it would be good to do a short plasma run and try things out before committing to a lasered batch. Getting it wrong with the laser is a very gutting experience, one I would rather not do again..
But my business is one where I sell products when they are completed, I rarely have customers waiting and do not take commissions, which is nice as there is no additional pressure...

Ricardoco, that is a cool bit of kit. I didn't know they were available, but can see the benefit and for a reasonable amount of money too. cheers for the link!

Danny