All sorts of things get said about rising damp, but most of it is wrong. Damp will find it's way through some really small areas, so 15mm would be no problem.
Back in the 70's, it was common practice to cut the DPM off flush with the slab, and not lap it into the DPC. Although only a small gap, it was found to sometimes allow moisture up into the plaster above and cause damp patches over the skirting boards. BRE started recommending 3 coats of Synthaprufe at the join.
Having said that, if it was on a cavity wall, a small amount of damp wouldn't be so much of an issue on the external skin.