Hi Trevor,
Yes, you are right, the camber is non adjustable according to the manual, but the pic below does give you possible causes for both castor and camber angles being out
However the real point is.....how far out are they, and are they both the same?
FWIW....from the manual:-
"Camber"
The relationship between the RH & LH camber angles is important - they should be equal. A difference of more than one degree will cause a pull to one side which will have to be corrected at the steering wheel and this leads to abnormal tyre wear.
Camber angle is usually slight - about 1 degree"
Personally I would not slot the mounts, although you might get the correct angle, the chances of them staying like that once the car starts moving the suspension around due to bumps and such like, is pretty remote.
John
1990 GTA Atmo, 2003 Jaguar X type 2.5SE Auto, 2018 Kia Picanto GT-Line-S 1.25