So what about putting the bearing back together
Well in the end it took over two hours of frustrating eye strain
Several times I was nearly there - down to the last ten or so balls - when something slipped and I'd have to start all over again.
On each side of the slider there are two bearing sized holes parallel to the rail that at their ends point at the tangent point on the rail where bearings are trapped between the rail and the slider. If initially the slider is held in the correct alignment, balls can be pushed down these tubes from one end, and they will emerge between the body of the slider and the rail where the bearing action is happening.
However if the slider is ever so slightly off alignment the balls escape and get jammed in the wrong places
Eventually I carefully measured the slider that was still intact, and put a small packing piece between the top of the rail and the under side of the loose slider to keep it in vertical alignment, with a tight rubber band around it to keep it together. Then it was a case of getting a few balls between the the rail and slider at the four tangent points to get the side to side alignment. Then 'just' a case of pushing bearings down the holes one by one until the holes and tangent points are full. Then quickly get the end cap on before it falls apart.
I probably repeated that process ten times
In one way the fact that the balls were magnetic was a help in picking them up with a tiny jewellers screw driver. But it was also a disadvantage, as, as you approach 'full tubes' several balls would decide to leap out and stick to the driver
If I had to do it again, not only would I pack the vertical distance, but I'd make up something to go round the rails to fix the side to side spacing, that could be gradually withdrawn as the tubes filled and the balls emerged at the tangent points.
Anyway eventually it worked, and my headache hopefully will subside over the next few hours