on other side of same stone wall as previous polypody
No interactions present.
Pinnae noticeably serrated. Unlikely to be P vulgare. Oval sori rather than round? Maybe Polypodium cambricum. Like you I'm not certain.
The sori have all passed their sell-by date, so are not much use. They need to be examined before they dehisce. Even then you probably need a microscope!
So that's not a problem, though what would one look for in fresh sori?
..and thanks for the help so far :)
I would suggest looking in a good flora that covers ferns as it will be explained much better than I can!
You can often still identify from dehisced sporangia - you need to count the number of indurated cells in the annulus. A fairly good rule of thumb is that you can usually see the amber to brown annulus even with just a hand lens in P vulgare, whereas the colour doesn't stand out in the other two. P cambricum has a shorter annulus - and the frond is usually more triangular in shape. Chris Page's 'Ferns of Britain and Ireland' has all the info.
The structure referred to is visible (just about) in the last picture at http://www.ispot.org.uk/node/292154
There are very few annuli left in you specimen illustrated above (maybe 3) and they are pale.
Sounds like p.cambricum can be ruled out then, and p.interjectum is most likely candidate, but I should be able to tell for sure when it spores afresh using your help (and the BSBI crib I've just located on the back of it).
It might not exactly match the expected sporing times (I think this is down to the unusually mild winter) but using the above help, this crib:-
.and a x40 dissector, I'm happy that the majority of the polypody in my immediate vicinity (Exeter SX9192), including those photographed above, are interjectum, with <10 indurate cells in the pale brown annulus, moderately serrate pinnules, moderately inflexed lower pinnules.
..and I need a dust mask for looking at sori under the dissector, watching them dehisce under the heat of the scope lamp was pretty - but has left me with a tickly cough ;)
Lat/Lng: 50.7, -3.6
OS grid ref: SX9192