if anyone can find a single successful UK case
I don't know whether there are any (and I'm not going to look), but this is spurious reasoning. A lack of (reported) cases does not indicate that a cause of action is not viable. To demonstrate the correctness of your position (which I agree is certainly arguable) you'd need to show cases where mountainbikers had failed to win when suing a landowner for injury. Otherwise, the absence of decided cases could indicate:
- that landowners are extremely mindful of these risks and have taken entirely effective pains to avoid them;
- that no mountainbiker has ever sued landowner (perhaps it's a matter of honour) or (improbably)
- that no-one has ever hurt themselves in a bicycle accident and thought "where there's blame there's a claim".
Remember also that we don't know an enormous amount even about Pinder v Fox really because there was a settlement not a judgment, and a confidentiality agreement. I hope you're right, of course, but as a lawyer your reasoning on this strikes me as reckless, and if I was advising the Duke of Buccleugh or the FC it's simply not the sort of thing I'd be writing.