Thanks to a leg-up from the facebook Pan-species Lister page I think this spot on the sorrel (Rumex acetosa) is the rust Puccinia phragmitis
It affects also Rumex crispa (Curled dock), and it is also visible on those plants on the reserve. In fact it's a very common sight. This seems to be the organism which has won the contract for dismantling Rumex leaves.
Aside from the benefit of identifying this one rust, I now am likely in a position to add double figures of species as the winners of the contracts to dismantle other species/genera/families make themselves apparent.
I have saved some and now I'm looking forward to getting it under the microscope. not that saving iti s necessary, if you consider just how much sorrel and dock there is!
(note; check Uromyces rumicis)