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Nice photos. Did you find any tiny white 'mites' on the body of the slug?I have seen them on a number of slugs and I would like to know what they are.
This all-black slug is probably Arion ater. This species is thought to hybridise with others but the appearance (and locality) suggest this is the "true" form.
White mites on slugs are likely to be the parasitic genus Riccardoella.
Amgueddfa Cymru - National Museum Wales
Thanks for the comments I had a bit of a look on the internet and it is Arion ater var. atra
This species comes in a wide range of colours including white subvar. albida
I have seen mites but I haven't got any photos and I don't know what they are.
With Best Wishes
Lat/Lng: 52.2, -3.4
OS grid ref: SO0560