Thanks! It isn't magnetic, so doesn't tick any box other than it fell from the sky! It was in our back garden and we are fairly hooligan free zone so I don't think it was thrown....perhaps a bird was carrying it? It was dusk and maybe they mistook it for a piece of meat? We are near Lincoln so I don't think the rock is local.
Birds - can't rule it out. They will carry stones: some for use as tools (to break eggs, but not in the UK methinks); some for use as nest decoration (eg. crows). It would still be rather unusual, however. (That reminds me: I've suffered from crows collecting my golf balls on a course in Canada.)