Saw this on another group - the way the branched pattern seems to be just on the surface of a curved, worn block suggests it's marks left by a modern encrusting organism of some sort. The last photo does look as if some of it could be internal though - hard to be sure.
I admit I've seen nothing quite like it before.
Thanks Tarquin, looking closely at it the marks seems to be 'in' the rock rather than 'on' the rock. I think I might take it to a local geologist for someone to properly inspect it as you'd suggested. Thanks!