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I'm still fairly new with millipedes, so not really confident with myself here...think your specimen could be Ophyiulus pilosus rather than Tachypodoiulus niger?
Many thanks; Perhaps more millipedes will turn up here in due course to give a better attempt at ID.
I had a look at this last night, but though I was sure it wasn't T niger, which is jet black with strongly contrasting white legs, I couldn't come to a positive conclusion as to what it actually was. It looks like this specimen has a strong pointed 'tail' which, taking overall colouration into account, points to either Ophyiulus pilosus or Julus scandinavius. At this point, Blower's key says the former is 'brown black, mottled ventrally' and the latter 'an even oily brown'.
Thanks for this reply. It is possible that the brown background is influencing the apparent colour of the legs; perhaps further specimens will clarify this.
Lat/Lng: 51.4337, -2.7624
OS grid ref: ST470707