I agree with Matt. If your landlord's got public liability insurance then he's covered by it, and if he hasn't he really ought to take some out. You could also perhaps point out that cachers are generally experienced walkers and users of the countryside, so are maybe less at risk than many casual users of his land.
You could also mention gc.com's disclaimer that a cacher seeks a cache at his or her own risk, though how that would actually stand up under English law I'm not sure!
Good luck with your negotiations!
Do not go gentle into that good night.
Rage, rage against the dying of the light. (Dylan Thomas)