hello lads, i found something that you might be interested in.......
i dont like to think that i am opening a can of worms.... again,... but i found an interesting 'fact' in a book called
British Sea Fishes 2001 by Dr Frances Dipper.
The book describes:
Shore Rockling: "mouth is small and the corners of the jaw hardly extend past the eyes"3 Bearded Rockling: "the mouth is large and reaches well behind the eyes"EUREKAAAA

the book also describes that the shore rockling is generally smaller in size - this compares well to the statement made by Al above:
shore rockling rarely grow over 30cms where as the 3 beardys grow up to around 3lb - but there is still ambiguity on the colouration and patterns on the skin of the fish. I have found that it might be safe to say that the 3b rockling has large dark blotches whereas the shore rockling has a mottled back... I THINK!

(bold statement) but then again, some have neither obvious blothes OR obvious mottling!
some fish will be immediately obvious as to what species it is e.g. a 45cm, 3lb, rockling with large spots is definately a 3bearded rockling!! but sometimes where it really is hard to tell which species it is then an experienced good guess will have to be forwarded by using combinations of all those characteristics as listed above.
if you are still not sure then leave it to the true taxonimists
in the meantime just eat them! - they are part of the cod family and in my opinion taste nicer than cod!!!!

hope this helps