Dermatobranchus sp.4 Photo by Andrew Taylor
Dermatobranchus sp.1 Photo by Guido Zsilavecz
Andrew would like confirmation that this is a Dermatobranchus.
The identification is correct. The species belongs to the family Arminidae, and Gosliner shows several Dermatobranchus species, all of which are still undescribed. The ridges of this species are distinctive, although there are also found on Dermatobranchus sp.1, which however lacks the darker markings, as shown on Andrew's photo, and has fewer ridges.
Nudibranchs of Southern Africa - a guide to the opisthobranch molluscs of southern Africa, T Gosliner, 1987.