"There are old large screw holes (plugs removed) where the old mounting
plates used to be. I will not be able to
drill holes in the correct positions for the new mounting plates because
the new holes would be too close to the old ones."
..
Thanks guys. For information, I don't have the original plates and the
(cheap and nasty) B&Q brackets have less than a centimetre of vertical
adjustment.

I can't reuse the old holes and the new ones would be very close to the old
ones; I'm worried about the safety, especially with the weight of a full
cabinet.. So the resin sounds like a possible option.

I'm also now considering making my own supporting plates. If I get some
large flat rectangular metal plates I could screw them to to wall securely
(with new holes away from the old ones) and have spacers to give a few
millimetres gap between plates and wall to allow the brackets to hook over.

Cheers.