FERN UNKNOWN - Bought from RHS that cannot identify it!

I bought a fern from the Royal Horticultural Society at Harlow Carr the other day.  It was the only one of its type that they had on display and when I looked to see what it was, it had no identifying name other than 'fern', which was not very helpful.  Anyway, I thought I would be able to identify it when I got home, by book or by surfing the net.  Wrong!  I am wondering if the fern is still very young and will display more easily identifiable properties when it ages. 

Fern

Anyway, I wrote to the Royal Horticultural Society and asked what was the complete name of this fern.  I was told that they buy in bulk from a supplier and would write to the supplier and let me know.  Several messages have gone back and forth and still the RHS has been unable to supply me with the name of the fern.   Shame on the RHS.  Imagine, the Royal Horticultural Society selling plants that they cannot identify to customers. 

Each fern frond has: a long fine brittle stalk (stipe); 6 pairs of opposite leaflets (pinna) on the blade, terminating in one leaflet; and each subleaflet (pinnule) is lobed. 

Parts of Fern

possibly (c) of FancyFrondsNursery.com