>If you dismantle the ballnut you'll probably find there's a ball in the wrong track, or bits of swarf in it.

Heh, no - I'll find myself on ebay looking for another one as it becomes clear that I'll never put it back together again..! That particular nut is very easy to get to as it sits directly under the table. Thanks for the tip re: bearing alignment. I did take my time putting it together and nipping things up progressively whilst moving the gantry from one end to the other. I'll have another look at it now. Despite the bit of binding when pushing the gantry by hand, it's still pretty easy to turn via the screw...

Cheers Jonathan.

Wal.