According to the mammal society (info here), pine martens and their dens are fully protected and must not be disturbed except under licence. Bats (rather more common in Norfolk than pine martens!) are similarly protected but I've read that they can be moved by suitably authorised persons, usually a member of a local conservation society, so I assume that a similar thing could be done with your pine martens. Presumably such persons either have, or know how to get, the appropriate licence.
Or perhaps they will just move on naturally once the young are weaned, which is after about 6 weeks apparently.