The entrance to this souterrain is in the ditch by the side of the road. It's marked on the map well into the field and the farmer had just dug a hole in the vicinity of the mark so I thought this must be it. Happily the farmer happened by and showed me where to look. He said that I could crawl in, that there's "a big cave in there". "Thanks all the same," said I.
The lintelled entrance is to the south. Immediately, after about a metre, there's a sharp right turn and the souterrain heads down. It looks like an ideal spot for a badgers lair and I had no torch. I would love to come back here with some proper kit and a companion less claustrophobic than I.