[Glenhuon (Bill)]

I think you have the standard Skywatcher rack and pinion focuser Jen. Correct?
Sounds like there is up/down movement in it like mine was.
Its not really noticable with just an eyepiece, but when you put the barlow in and eyepiece on top it becomes a nuisance especially if you are focusing with the focus tube a fair bit out.
You might have to fit an extender before the barlow so the outward travel is not so great. Either that or fit a Crayford and extension like I did.
The other suggestion of having the eyepiece/barlow out about a centimetre or so would help too.

Bill