Hymenoscyphus repandus is saprobic on dead stem of herbaceous plants. According to NBN, this species has not been recorded here before but there is a small cluster of records from the north-west highlands of Scotland. Probably not uncommon just under-recorded. The main hosts are willowherbs (Epilobeum spp.) but other genera are also used. Dennis (1978) gives an greater list of hosts but no mention of Epilobium and records it as common.

References: 
Dennis, R.W.G. (1978). British Ascomycetes.
Ellis, M.B. & Ellis, J.P (1997 new enlarged edition). Microfungi on Land Plants: An Identification Handbook.