Although the FSC guide (page 8) says that the mites which run around on slugs are Riccardoella limacum, they are actually Riccardoella (Proriccardoella) oudemansi (Thor, 1932). Dave Gibbs found this paper by Zabludovskaya & Badanin which demonstrates that the mites on slugs are R. oudemansi, not R. limacum: http://mail.izan.kiev.ua/vz-pdf/2010/2/06_Zabludovska-Badanin.pdf
Dave used this ID paper to check the identity of some mites on a Tandonia budapestensis slug in his garden: http://bionames.org/references/9509117b78453fbd0f762c97873c642f
They were R. oudemansi, as expected. Annotated photo here: http://www.brc.ac.uk/psl/photo/riccardoella-oudemansi-slug-mite
Also note that according to Fauna Europaea, R. limacum is absent from Britain. I'm not sure how much weight to give to that - mites on snails in Britain could be either R. oudemansi, R. limacum or other species. Worth checking.