Puccinia cnici-oleracei is a rust fungus that affects the leaves of Marsh Thistle (Cirsium palustre) in VC55, and on Melancholy Thistle (Cirsium heterophyllum) in other areas. There is no host alternation, and only the dark-brown telia are found, on slightly thickened yellowish spots.  

Also occurs on other plants in the Asteraceae (e.g. Yarrow, Ox-eye Daisy) but is probably not a gall-causer of these species (ref. Redfern & Shirley 2023).  Some authors consider the rust on Achillea to be a distinct species, Puccinia millefolii.

host

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments. On some hosts, microscopic examination may be needed.

on Thistles and Yarrow