Thanks for all your answers, the main problem is that the units don't touch the wall at the back when levelled, because the wall or floor is out.
There's a gap of around 1mm at the back of the worktop, and to add to the fun the wall is concave.
This is with only a tiny amount of overhang at the front of the worktop, which ideally I want to increase a bit.
Having read your answers maybe the best way would be thick tiles to take up the gap?
Or moisture resistant plasterboard, then tiles?
I think glass would look good but it's a rental property so not sure of the safety aspect.
Thanks