From my English correspondent RJP, this tradeperson’s van, photographed on the street:
Flat Boy Skim is a bit of complex name play on Fatboy Slim. Well, you have to know who Fatboy Slim is, something many people do not. And then: what might Flat Boy Skim have to do with plastering? For that, you have to know something about the technical jargon of plastering (which I did not, until I looked it up; well, I correctly noted that a good plastering job should be flat — smooth — and I assumed that boy was just there for the name play, but skim was a mystery).