Confidence: It's likely to be this, but I can't be certain.
Notes: The slightly pink colour, with the pair of narrow very dark stripes along the mantle and the body, the fairly short tentacles, and the lack of a prominent keel, suggest Lehmannia valentiana. Limax maximus would usually have spots and marbling on the mantle rather than regular stripes, and usually has more dark markings on the body. It also has more coarse tubercles a more prominent keel and longer tentacles. L. valentiana is often gregarious in compost bins and plantpots, whereas L. maximus is usually solitary.