Tiddy CNC.

[Jonathan]

[...] thoroughly cleaned the rest of the rail with degreasant and elbow grease. The rail came out really clean, the bearing I had removed ran rock solid and smooth over the rail and once bolted back in the gantry is noticeably smoother. I'll do the other side next day or so. I have already got the right type of grease for future lubrication

Just to be sure, did you put any grease or oil in the bearing before testing again and finding it to be 'smooth'? If not, then (at risk of stating the obvious) do put some in before going further as running without any lubricant (which is presumably the case if you've cleaned it thoroughly) even for a short time will reduce the life of the bearing. Hard to say by how much, but there's no need to risk it.

Edit: You might want to check the ballnuts for debris whilst you're at it.