I see where you're coming from with the centipede idea...
I suspect not, but I can't offer a good explanation either.
From the way one of the dark lines wraps around the stone (in 003.JPG), it suggests the dark part may extend through the stone in a vein-like manner (the two 'arms' looking as they do just because of the way the surface of the stone intersects it).
The smaller-scale structure/texture is intruguing. I could believe it is fossil, ie. of animal origin (less likely, plant). But I wonder about it also being due to iron (ferric) minerals permeating micro-fractures in the flint. As I write that, I don't really 'buy' it. There is something fossil about it.
The flint also has banding (only just visible), visible in all photos except 003.JPG. I am sure that is unrelated to the main object of interest. (Re banded flint, see http://www.nhm.ac.uk/natureplus/servlet/JiveServlet/download/8377-2960/Banded%2520flint.doc)
Need other opinion(s)!...
Thanks Mike for your input. My photographs could do with some improvement. Its not clear in the photos, but there are bivalve like radiating lines across the whole structure starting from the corner where you mentioned the long "antenna" does wrap around below the plane of the main structure.There appeasr to be bilateral segments along the main body as well (clearer on the real thing).
It seems soft body preservation is extremely rare, from this era, but could the creature have been inside the shell and hence unusually preserved.?
I'll try and post some clearer pics tomorrow!
I think we're talking about the same thing: your 'bivalve like radiating lines'. They are what I was referring-to when mentioning banded flint. Pieces of banded flint often look convincingly 'fossil'. I am sure they are just a distraction from the main ID here.
I see what you mean about the bilateral symmetry, but (to be sceptical) I could be more convinced.
I don't know what you mean by 'inside the shell'...
It would be very interesting to saw through the specimen to get a cross-sectional view, then polish the surface. But that (probably) wouldn't be easy for you, unless you have access to a local geology club, university earth science dept., etc. And you would need to be very careful when sawing flint, because of the risk of it breaking and throwing off sharp fragments.