Thanks guys, I have considered most of those ideas so its good to know im thinking along the right lines.

Kip
did you mean 1mm or 1 m? What are rod less cylinders(probably regret asking that...) are they air driven?

Basically it's just more sliding joints to cause a problem.

If the gantry doesn't need to move during a job then it doesn't need precision rails, it just needs to be adjustable with clamps to secure it in place.
this sounds the most favorable, Think I saw someone on here suggest multiple holes on the Z to gain more bed height, perhaps allow fixings on the end at 50mm increments and then move it when neccessary.

Just a quick thought, instead of moving the gantry up to give you more room why not move the bed down??
Not keen as there will definatly be a rotary table going in for 4th axis

Any one used dual X axis to extend travel?, ie. Moving gantry and moving table.