Apologies for the delay in response but my weekend was wiped out by a family emergency and am just recovering from the aftermath.
Re natural feature.
Sec 13 which amends the Occupiers Liability Acts states:
6(A) ...an occupier of the land owes (subject to subsection (6C) below) no duty by virtue of this section to any person in respect of?
(a) a risk resulting from the existence of any natural feature of the landscape, or any river, stream, ditch or pond whether or not a natural feature, or
(b) a risk of that person suffering injury when passing over, under or through any wall, fence or gate, except by proper use of the gate or of a stile.
(6B) For the purposes of subsection (6A) above, any plant, shrub or tree, of whatever origin, is to be regarded as a natural feature of the landscape.
(6C) Subsection (6A) does not prevent an occupier from owing a duty by virtue of this section in respect of any risk where the danger concerned is due to anything done by the occupier?
(a) with the intention of creating that risk, or
(b) being reckless as to whether that risk is created.?
Re CRoW applying to dug entrances, I claim dug entrances are a means of access, see
http://ukcaving.com/board/index.php?topic=17137.msg225933#msg225933 but I fear only the courts will resolve this question which will depend upon the specific case.
Re land owner liability re digs, my line of thinking is that it will depend upon what was agreed between the diggers and the land owner. (And that could be either a verbal or written agreement.) If the diggers agreed to fence the dig, then the state of the fence is the diggers liability and more importantly, their continued responsibility for keeping it in a satisfactory condition until they die or end the agreement. My expectation is that under most casual agreements, one might consider the diggers as 'occupiers'. (After all they are depriving the land owner his normal right of use of that patch of land.) And as a consequence, the SSSI PDO would apply to the diggers, as well as the land owner since W&C Sec 28E does say "The owner or occupier". I am not sure if the diggers acting as a contractor would materially change this situation, though it would bring in other legal demands, notably H&S. If no agreement was obtained, then the diggers could be construed as squatters and hence occupiers. However following the squat / dig, I think the liability would then fall back onto the land owner for not removing the risk from the dig (who could then sue the diggers for the expense in doing so).