I was quite pleased with this little guy but it seems it's just a brown variant of the black slug (ref. Field Guide to Land Snails GB & NW Europe). Does seem to be carrying one of those little shells that someother species have, so maybe I'm being too hasty, although that itself seems rather conical. Is it really a shell?. Didn't notice it in the field.
From another angle a mobile phone pic from the day seems to show a shell which may have been dragged squint.