Rattle, As will have noted, this has often been a contentious issue and there is usually some dispute amongst contributors here!
I have been flying helicopters under both military and civilian rules for quite a long time (since the late 1970s); also under police ANO exemptions.
The best advice I can give is that it appears to be a grey area in UK law, so it's always best to ask in advance. If you do that, you will at least gauge the likely response of the landowner. Even with his/her prior permission, you will be expected to ensure 100% that no-one is endangered or upset, or put to expense, by any actions of yourself.
If you also inform the local police in advance and obtain an incident number (a simple routine phone call; don't abuse the 999 system to do it), you are covering your backside and hopefully preventing the police from unecessarily sending resources to investigate an incorrectly reported aircraft accident by a well meaning but poorly informed member of the public. The local non-emergency police numbers are published in the back of the Pooley's Guide or can be found by an internet search or in Yellow Pages.
Hope this helps.
