It needs securing to the wall at the top to stop it moving: have you done
that? Usually some small L-shaped brackets.


there's no way of fixing anything to the bath, unless there's a specific
clip/bracket for the job that clips under the lip and is screwed to the wall
?

I could drill through the lip and screw it to the wall that way but don't
want to take the risk of ruining the bath (SWiMBO would kill me (c; )