Approx 50mm long
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I didn't know slugs could be so complicated! Looked on E of L at yellow slug and saw a dark coloured one, so obviously I shouldn't jump to conclusions by the colour. I will put images on here and let the more learned people help me. Thanks Jason.
This is an species of Arion rather than Limax - Limax slugs have a keel along the top of the body, running from the tail end about two-thirds of the way up to the mantle (the 'saddle' on the front half of the body). Also, the mantle in Limax has fingerprint-like ridges on it, rather than the more granular texture shown here for Arion. Also (but not visible in the photo because it is on the right-hand side of the slug) the pneumostome ('breathing hole') of Arion slugs is nearer the front of the mantle than the back (vice-versa for Limax).
At 50mm this would only be about half-grown for Arion ater or one of its close relatives, so not possible for me to suggest a species.
See further discussion of the difficulties of Arion slugs at:
Entomologist and biological recorder
Lat/Lng: 53.7, -0.4
OS grid ref: TA0730