A population was studied in a stream in Bristol by Hartley (1962). The stream was subject to sewage contamination, and this chimed with the allotment in Gray, which was treated with "night soil". (I can say that in 1922 most houses in south Tyneside did not have flush lavatories, and that a household's excrement was removed at night through a door beneath the lavatory in the house's back yard, in a terraced row of houses' back lane. It was then distributed for agricultural and allotment use). Hartley suggests the leeches have a lifespan of 4-5 years, and that the adults spend most of their time away from the stream. Reynolds (1996) also notes the leech's residence in drains and sewers.
Warwick (1960) refers tantalizingly to an "unpublished occurence" of T. subviridis in Glasgow.
There is nothing else, so far as my limited searches go. If and when I find and photograph any more specimens, I have to hope that any researchers interested in T. subviridis find their way here...
Gray, R. H. (1922). The occurrence of a Leech (Trocheta subviridis) in an Allotment. Parasitology, 14(3-4), 320-321.
Hartley, J. C. (1962). The life history of Trocheta subviridis Dutrochet. The Journal of Animal Ecology, 519-524.
Reynolds, J. D., & Tenison, M. (1996). Trocheta subviridis Dutrochet, a Leech New to Eastern Ireland. The Irish Naturalists' Journal, 25(8), 302-302.
Warwick, T. and Mann, K. H. 1960 The freshwater leeches of Scotland. Ann. Mag. nat. Hist. Ser. (13) 3, 25-34.