The 1984 Occupiers Liability Act was not created to establish any new duties or responsibilities (contrary to what some think). Instead it was intended to clarify an area of common law that had become confused, namely the duty to trespassers on private land. It is a piece of civil law; it gives rise to torts for which you can sue for not criminal liabilities which you can be prosecuted for.
It does the following:
Section 1, it establishes that there is a duty of care, which is owed to "persons other than visitors", who will predominantly be trespassers but this also applies to anyone exercising rights under various laws granting access to the countryside and anyone accessing a private right of way, but does not apply to anyone using a public right of way in which case the common law rules apply.
Under Section 1(3) of the Act, the duty is owed when the occupier is aware of the danger, or has reasonable grounds to believe it exist, and knows or has reasonable grounds to believe that the trespasser is near or may come to be near the danger and the risk is one which an occupier may reasonably be expected to protect visitors from.
So the first thing you need to do is establish if any such risks exist on your property.
Just because an area is fenced off does not mean that the occupier knows that a risk exists. In the case of White v The Council of the City and District of St. Albans [1990], where the claimant had taken a shortcut across the defendant's fenced-off land and fell into a trench. He argued that the fact that the defendant (The Council) were taking precautions to stop people getting into the dangerous area meant that he believed somebody was likely to do so, and was therefore liable. The Court of Appeal rejected this argument, saying that just because a defendant had tried to prevent people entering dangerous land did not mean that the "reasonable grounds to believe" have been satisfied.
Section 1(4) establishes the duty, that the occupier “takes such care as is reasonable in all the circumstances of the case to see that the non-visitor does not suffer injury on the premises by reason of the danger concerned".
So you have to take reasonable measures to keep people away from high risk areas, eg fencing keeping places locked up etc.
If the duty of care is breached and the trespasser suffers injury, unlike the Occupiers' Liability Act 1957, the 1984 Act only allows an injured trespasser to sue for compensation for death or personal injury, rather than damage to any personal property.
Signage can also be used as described in Section 1(5) of the Act. It states that the occupier discharges his duty "by taking such steps as are reasonable in all the circumstances of the case to give warning of the danger concerned or to discourage persons from incurring the risk". However, simply providing a warning sign is not enough; the sign must be clear enough to ensure that the risk is obvious to the trespasser. Whether or not the warning sign makes the risks obvious is dependent on the trespasser; warning notices may be considered inadequate for children, who may be either unable to read or unable to appreciate the danger.
Once everything reasonable has been done to prevent injury to the trespasser, then you can assume that the trespasser has accepted the risk under the doctrine of volenti non fit injuria, which is described in Section 1(6) of the Act, which says that "no duty is owed ... to any person in respect of risks willingly accepted as his by that person".
As far as Section 3 of the Health and Safety at Work Act is concerned, all the above measures (ie everything that poses a risk is closed off and inaccessible and signage provided) should stop you getting prosecuted under the Act. It would not be public policy for the powers that be, to prosecute someone for injuries to a trespasser, unless there had been a gross lack of H&S management. I believe that couple of years ago the owner of an abandoned factory was prosecuted. They had left a tank of paint stripper in a factory building, without any attempt to make it safe or to dispose of it. A nine-year-old boy climbed onto to the roof to reach the ball when the roof of the building gave way and the boy crashed to the ground below, landing in a tank containing the chemical once used to strip furniture.
I can’t confirm what the final outcome was.