Epilobium

Jäger, E.J. 2011: Rothmaler, Exkursionsflora von Deutschland, Gefäßpflanzen: Grundband, ed. 20Meikle, R.D. 1977: Flora of Cyprus 1Stace, C. 2010: New flora of the British Isles, ed. 3Tutin, T.G., Heywood, V.H., Burges, N.A., Moore, D.M., Valentine, D.H., Walters, S.M. & Webb, D.A. 1968: Flora Europaea 2
The key is based on the cited sources. It needs further testing. Hybrids (partially or entirely abortive fruits!) may complicate determination.
1Lower leaves alternate; flowers numerous in terminal bracteate racemes, somewhat zygomorphic; petals entire or obscurely emarginate
1'Lower leaves opposite; flowers solitary, axillary, actinomorphic; petals bilobed at apex
2Stigma 4-lobed (caution: lobes sometimes erect)
2'Stigma clavate, without lobes
3Petals 10-20 mm long; leaves sessile, amplexicaul
3'Petals 4-8 mm long; leaves not amplexicaul
4Stems closed with spreading hairs; lower leaves sessile or very shortly petiolate
4'Stems clothed with crispate hairs; lower leaves distinctly petiolate
5Petals (7-)11-13 mm long; stigma usually elevated above the anthers at anthesis; anthers 1.5-2.2 mm
5'Petals 2.5-7(-8.5) mm long; stigma surrounded by anthers at anthesis; anthers 0.7-1.1(-1.3) mm
6Leaves oblong, more or less decurrent on to stem-ridges, light green; midrib on lower side of upper stem leaves glabrous
6'Leaves mostly oblong-lanceolate, shortly petiolate, not decurrent, greyish-green; midrib on lower side of upper stem leaves hairy