Have finally settled on a single motor, single leadscrew, but with extra bearings and rails under the gantry. I know I know... Pictures..! I will try to put a couple of sketches together tonight and post them..

Have listened carefully to everyone's if, buts and maybe's about dual driving and have to agree that a single drive is much less prone to errors, and belts are, in my experience, another added layer of mischief..

But I do think it important to stabilize the gantry as any yaw will not be acceptable. To this end I have come up with a way of using additional track and bearings (as these are so cheap) to give extra guides to the gantry...

Pics later..!