I know they are difficult but this leafy hawkweed looks very distinctive. Photo take in North Somerset 30/7/12, only one flower just starting to open so a late flowerer. on old railway siding. If not possible from this image please let me know what features i need to get shots of next time i visit.
yep, these are tricky little so and so's! As you possibly know, Hieracium are almost entirely obligate apomicts (which makes the whole idea of the species concept, especially microspecies, as in the case of these, even more spurious...) so they can only be keyed out to section by all but the most fanatical... (Of which I am not one!)
According to Stace the key features to look for are -
are basal rosette leaves still present at flowering?
do the middle leaves clasp the stem at all?
do the basal leaves have a petiole, however short, or not?
After those features the other that help are whether the scales on the outside of the flower head recurve or not, whether the hairs on the flower head are glandular or not, the colour of the fresh styles and the colour of the ripe seeds.
From what I can see from your pics my best guess would be section Alpestria, but as is often the case with these things I can't give a definitive id with the info available, sorry.