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This is a tricky one - I believe you are correct. It's definitely a Porcellio sp.. The contrasting dark head, dark central stripe with (barely a hint of!) adjacent longitudinal yellow flecks all back up your ID. The pigmentation pattern is too neat and symmetrical, and the body tubercles are not prominent enough, to be P. scaber. I suspect this is a poorly pigmented and/or immature P. spinicornis.
BMIG website: www.bmig.org.uk (centipedes, millipedes & woodlice)
BMIG Newsletters ~ www.bmig.org.uk/view/resource/bmig-newsletter
BMIG Bulletins ~ www.bmig.org.uk/view/resource/bmig-bulletin
Thanks Steve. It had me puzzled for a while. I have found this species in Bristol but never in Derbyshire. It appeared indoors in the bath! But the garden habitat outside includes loose mortared walls and bits of limestone which seems the right habitat.
Lat/Lng: 53.246955, -1.910957
OS grid ref: SK060722