Advice on levelling base units

I have spent an afternoon or two struggling with trying to level a base unit. It is made up of a 500mm and a 400mm unit. They are screwed together to make a single unit. The problem is the floor is uneven (we had it levelled earlier this year but for various reasons they didn't do a good job, it is a concrete floor and they wanted to dig the entire lot up, which would have been very expensive).

I almost got it level the other day, but then I discovered my spirit level wasn't that accurate....

Advice on levelling base units