Haemopsis sanguisuga, Herpobdella octoculata or Trocheta subviridis?

The problem with tweeting about one's allotment adventures in an hour or so after they occur, is that if you've made a proper Charlie of yourself, for example by crushing a blameless leech beneath your Dr Marten's boot, you're revealed to the entire twitter-sphere - in particular your fellow allotment tweeters - as the aforesaid proper Charlie.

This afternoon, with a combined sense of contrition and curiosity, I've been researching this whole flatworm and leech business. It's now pretty clear to me that this was a leech, more of which in a moment.

But I have learned this about flatworms, there's no evidence that they are causing damage to earthworm populations in the UK. Fear of them appears to be another aspect of the hysteria regarding almost any "invasive" and "non-native" species. Grey squirrels and their persecutors are perhaps the sharpest example of the phenomenon in the UK.

A whole host of beasts prey on earthworms in addition to the flatworms: hedgehogs, foxes, moles, frogs - and, for goodness' sakes, bloomin' leeches. Naturally, even in our anxiety to ensure a good earthworm population for the benefit of the soil and all that grows therein, we're not going to attempt to exterminate everything that uses them for food. Indeed, the existence of flatworms and other predators could be argued to be indicative of a healthy earthworm population.

My best guess now is that the poor beast I stamped on the other day is a carnivorous, amphibious leech, which has got under the plastic sheet in search of earthworms and perhaps slugs and their eggs, (another good reason to have left it in peace). I've been looking at photos online, and my best bet is Trocheta subviridis. I hope to find more of the same when I lift the opaque tarpaulin in early autumn, a job I'll undertake with great care, a few feet at a time. Hopefully, I can get hold of one to examine it more carefully before releasing it.