The LX200 is working again.
To be honest I'm not sure why.
I:
1) tried a different RA motor (no luck)
2) switch xylinx chips - worked!
3) put the old RA motor back - worked!
4) put the old xylinx chip back - worked!
.... so, it's working now with exactly the same components it had a week ago when it was working. Only things have been re-seated, notably the xylinx.
I'm unsure how much I trust it. Running automated there's a risk it does dangerous stuff during the night when it breaks again. I suppose if that happens the worst that's going to happen is it'll damage it's self, leading to the same conclusion as otherwise: buy a new mount.
A new mount is still looking very attractive. But, at the same time, this was running so nicely a week ago.
Time will tell what happens next.
Roger.