In reply to Mark Warnett:
My understanding is that landowners could indeed be sued by a climber who injures themselves on their land. Equally the climber could sue Queen (royal or band), Mick Jagger or Putin. All of these cases would have about the same chance of success though...
If you engage in a sport, you consent to reasonable injuries (volenti non fit injuria); in a climbing context this would be any any climbing injury? Obviously if the landowner booby-trap the cliff so the whole thing falls down when you get to the belay ledge, they will be responsible, but otherwise it is widely understood that rock is natural, unpredictable and inherently risky.
It is true, as you say, that you have a duty to take reasonable steps to protect trespassers, but this has been overridden by volenti non fit injuria where the trespassers have willingly exposed themselves to risks. Wikipedia gives three examples, all decided in favour of the landowner. First, a girl who broke through 'adequate' fencing onto a railway and was then hit by a train; secondly, a student who dived into the shallow end of a closed swimming pool and was injured; and finally a man who dived into a shallow lake despite 'no swimming' signage.
The other case which I always find interesting is 'TRUSTEES OF THE PORTSMOUTH YOUTH ACTIVITIES COMMITTEE (A CHARITY) vs POPPLETON' - where an inexperienced climber, attending a bouldering wall with his more experienced friends, did something stupid copying those friends (jumping from the wall onto a girder), fell badly and became tetraplegic. The wall was (eventually) not held responsible - despite less-than-perfect signage.
http://www.bailii.org/ew/cases/EWCA/Civ/2008/646.html
The first line from the report:
"Adults who choose to engage in physical activities which obviously give rise to a degree of unavoidable risk may find that they have no means of recompense if the risk materialises so that they are injured."
<end armchair law>