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The yellow stripe down the keel is a strong indication of Yellow rather than Irish Yellow Slug, and the speckling on the mantle isquite small, again suggesting it's not Irish.
....confirmation Brian - has the yellow reached London?
Till the 1980s, almost all mainland British populations were Yellow. Then, a few populations of Irish were noticed - Liverpool was a stronghold. In the last 10 years, Irish has been turningup in lots more places - it's common in Cambridgeshire, has almost replaced Yellow in parts of Yorkshire which I surveyed in the 1980s-90s, and I found it abundantly in rural Perthshire this autumn. I'd guess you have both in London by now.
Lat/Lng: 51.4872, -0.1235
OS grid ref: TQ303781