I wonder if it's either Cylindroiulus londinensis or Tachypodoiulus niger? I think they're quite hard to tell apart - it's all down to the telson, or rear end! The apparently rounded end of your specimen suggests Cylindroiulus.
Thank you for this. I had no idea that the rear end of a millipede could be so important! I found a website that had drawings showing the difference in the telsons of the two species and I compared that with another photo I had that showed the telson better - I think you're right and it's Cylindroiulus. I've attached the other photo, cropped to show the telson.