I know that straying into the law always causes a massive debate, but the truth is that signs afford very little by way of defence. That's not to say don't bother with them, but on their own they are pretty meaningless. In most cases this is nothing to do with any legislative requirements of the Health and Safety at Work etc Act 1974. But we have had that debate before. Whether or not you agree, there is very little likelihood of a criminal prosecution being brought against anyone who owns a high seat should someone fall off. What people are really looking to satisfy here is some notion of civil law, possibly just under a general common law duty or under something like the Occupier's Liability Act.
So, what's the point? Well go with the signs, sure, but the wording is not dictated by law. In all probability the words Private. Keep Off would be good enough. The reality is that in order to pursue a civil action against you should someone fall off, they would have to prove that a duty was owed in the first instance - well if this is in a publicly accessed place, the plaintiff could probably swing that one, but then that the duty was breached in a negligent fashion leading directly to the injury. If you have a rickety old rotten seat or tower, then the fact that it was not safe for YOU kinda seals the deal for them. But if the seat was fine and they just pillocked about on it, then you have not failed.
They key to proving this is really inspecting the seat regulalry (no I am not defining what 'regular' is) and actually keeping a record of condition. Now I have seats up in Scotland that I struggle to find - and I put them up. So I don't have signs on or inspections. But if I were somewhere really quite public, like in the middle of Cannock Chase, I would take pics and note defects. I may even go as far as removing ladders on some structures, or at least locking them off with boards. There really is no hard and fast here, it's all context dependant.
No doubt someone will say 'well I have been told this or that' and that may be right in that situation. But you have to act according to each situation. Common law is stored in case histories, not in books of rules.
So in summary
This is less Elf and Safety and more civil law
Signs are only part of it and the actual words aren't especially critical (not everyone can read)
Condition of the asset is the most important bit and crucially being able to show that you cared for them
If they aren't safe to be used by anyone, then do something about it.
Hope that has helped rather than confused anyone. As part of my role, I help in defending our business against claims that run to 6 figures so I know how the system works and what the courts look for in a defence. And trust me, it's not a well worded sign. Actually it could be a badly worded sign that is the undoing. Certainly don't put 'Dangerous, keep off'
Chris