Hi Beezie,
I don't have any 'hands on' experience with this particular lathe but I have come across this type of problem with other lathes so this may apply to yours. I have also looked at the parts listing from the Axminster manual download but it's not clear enough for me to sure about this either. I have used the parts listing to highlight the bits.
On many lathes the quill locking screw (18) engages in a slot in the quill (13). This serves two purposes - it stops the quill rotating as you wind it in and out, and it prevents damage to the surface of the quill, which needs to slide freely iin the housing. Unfortunately the parts diagram shows a flat/slot on the top of the quill but no slot on the side adjacent to the locking lever.
If this locking screw has been taken out or even just undone a long way, it could be that the quill has rotated slightly so now, each time you tighten it, you damage the surface, causing it to bind. Another possibility is that the quill doesn't have a slot but would have a plug of soft metal between the quill and the locking lever to prevent damage. If that's not there, the quill will suffer. It would be easy to check this, just remove the quill completely and look for any surface damage. If that is the case some careful use of abrasives might fix the problem or you could ask Axminster for a replacement quill.
Hope that makes sense and, whatever the cause, that you get it sorted quickly.