Thanks for this Jazzcnc, good advice.

The 'quick method' posted above in the youtube vid, didn't really work for me because getting a 2.5m long ballscrew vertical and then being at the top, in a convenient place see clearly into the nuts and spoon the bbs in seemed troublesome...

So yeah, after cleaning everything up with lots of brake cleaner (it was reasonably grubby in there), I used the grease dabs to 'stick' the bbs in place. I'm surprised at how well that worked! :D Did it over a
cardboard box lined with paper towel. Miraculously, number of bbs lost = 0.

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Upon reassembly, I realized how poor the 'wiper seals' are. Just pieces of hard white plastic that don't seem to fit very well, and that get slightly deformed by the two grub screws holding them in place. They leave a thick layer of grease from inside the nut, very much ALL over the screw outside the nut, where it will attract dust & debris. Has anyone made their own seals for these ??

Anyway, it's now MUCH less gritty and rolls well. Still has noticeable lateral play mind you... As you say Jazz I think I should cut my losses.
I messaged Fred on Aliexpress. I was considering a double-nut to remove backlash, but looks like he can't do that on 3232 only 3210. Any experience or opinions on using a double nut for such a large x axis? When I asked about c5 screws Fred mentioned 'TBI' ground and TBI C5 Rolled' ballscrws. I'll admit, I have no idea what "TBI" means - anyone?

Ok thanks again guys.