Actually not all ferns are difficult to identify, some are quite unique in appearance and cannot be confused with any other. The hard fern (Blechnum spicant) is one such species and is not hard to identify at all! The fronds are very thin and give a 'serrated' appearance, a bit like a double sided comb perhaps? No other Britsh fern is like this and so can be named on sight.
It prefers acid soils and is found mainly in the east of Dorset in shady woods and hedgerows; I see it most frequently along ditches and stream sides. it apparently also occurs on rock ledges but as I don't climb rock faces I can not confirm that!
Some of the fronds die back in winter but others stay evergreen and so you can find the hard fern easily any time you are out and about in suitable habitat.