I have a small hand wash basin ready to be finally fitted. I have it just
fitted onto its studs at the moment and is sags slightly from back to front.
Possibly due to plasterboard being a bit distorted by previous wash basin. I
have injected Pink Grip into the space between the plasterboard and the
breeze block now.

I was considering applying silicone to the back of the basin - tightening it
to the wall whilst wedging it level. However, I guess this could make future
removal a problem and was wondering about using plaster. Apply a few dobs -
tighten it whilst it is wedged level and wait for it to set. Then seal
around edge with silicone.

What are your thoughts?