In reply to mbh:
The film company will almost certainly have obtained permission from the landowner (and will probably have paid a fee) so it is 'theirs', at least temporarily. That doesn't necessarily give them the right to exclude others from what is open access land under CRoW.
There are provisions which allow temporary closure of CRoW land, but according to government guidelines risks to public safety from natural features isn't a valid reason. The guards are possibly acting unlawfully if they are turning people away for this reason, whereas if they were to say it is because of filming it would perhaps be OK.