I got the Meguiars Smooth Surface Clay Kit. For most of the body I was gentle just wiping it over the bodywork, very suprised at the amount of dirt it was fetching off. But went back a couple of times to a couple of areas (boot, lower door, wheel arches) and was a bit more agressive focusing on literally a spot of tar. A lot of them came off with a bit more attention, but there were a couple that decided they wanted to stay on.
I could not see any marks on the on the paint, and always careful to make sure there was plenty of lube on the body. I might try another clean this weekend and see what happens. If it still refuses to shift maybe a bit more agressive clay and be really careful with it.
Cheers