Should really know the names of this one :# tend to call them orange skirted slugs to the youngsters.
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There tend's to be more black ones in the north of the country and lighter in the south ,so you have done well to have both together.They do not cause plant damage as they feed one dead and rotten foliage.
Lat/Lng: 53.857655, -3.0235745
OS grid ref: SD327406