Makes sense... my dad lives over in England, so I'm not sure what the rules are there, but probably very similar (after all in theory these things are decided by common sense ;)
Purely speculation, but maybe it matters whether it's a free-standing structure or fixed with a bit of concrete bedding. There might be local laws about whether a rock can even be placed at all (I don't think we're allowed to put anything - other than a mailbox - within so many feet of the roadside here even though the property line extends right up to the edge. I think that's a DOT law, not a town planning thing)
Either way, if it were me I'd seek advice from local authorities first as they can surely advise on best course of action according to local rules (and/or get things fixed as necessary)
cheers
Jules