I found this half-buried in the dirt whilst tramping round the old Harford Farm site. I'm assuming this is an old medicine bottle. As well as being the area of a former farm and Anglo-Saxon burial ground (covered in previous posts), it is also next to an old (now capped) landfill site. I've spoken to a chap who said his brother used to go exploring there as a kid and collect any medicine bottles that had been buried there.
However, as I'm still on a big M R James trip, I'm convinced it is in fact some cursed artifact of Lovecraftian evil. A witch bottle, at least. Unlikely, but still.