This is common nationally but seems to be scarce in Scotland. This is the first record for the Outer Hebrides (2025). The main host is great willowherb (Epilobium hirsutum) but most species in the Epilobium genus are used.

References: 
Termorshuizen, A.J. & Swertz, C.A. (2011). Dutch Rust Fungi.
Wilson, M. & Henderson, D.M. (2011 reprint). British Rust Fungi.