A white slug 22-25mm long
No interactions present.
John.. A Marine slug? I don't think so.
Change it to Gardens and Parks via Edit
THREE MARINE PROJECTS
http://www.naturespot.org.uk/species/netted-slug - looks close
Amazing - where, in the UK is it unrecorded? http://www.gbif.org/species/5781277 - an impressive map.
May come back to agree.
Lat/Lng: 53.3, -6.3
OS grid ref: SG1491