Is a species possible? About 4cm long.
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If it really is 4cm in length it could be Lithobius pilicornis - locally common on south coast Cornwall, Devon, and occasionally around houses/churchyards in towns inland, sometimes indoors in cellars (!). L. forficatus rarely exceeds 3cm.
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
It's a while ago now, but I'm usually pretty good at estimating lengths. The location does not really fit with what you've said, though, being found in the Vale of York, under a log in a school's copse conservation area. I'm happy with the genus, to be honest.
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Thanks for the ID, Geophilus. We can now add this one to the list of species of different creatures found at school, which is ever-growing!
Lat/Lng: 53.9, -0.8
OS grid ref: SE7948