Confidence: It's likely to be this, but I can't be certain.
Notes: Tricky. I think it may well be an adult Lehmannia valentiana - it's usually pink with the two dark stripes, and occasionally additional mottling. If it's crawling actively, the shorter tentacles suggest Lehmannia rather than Limax. And L. valentiana is often gregarious in compost bins, whereas L. maximus isn't.