In reply to Woker:
The situation appears more complicated than it really is because the SSSI designation is deliberately vague. This is to enable a vast range of habitats and geological sites to be encompassed using a single umbrella term; namely 'Site of Special Scientific Importance'.
However, each SSSI possesses unique attributes which have led to its particular selection and hence the range of actions which might consider to threaten the 'scientific interest' (or, to use the bureacratic jargon, 'Potentially Damaging Operations' (PDOs)) will obviously differ from site to site.
So it's not a 'one-size fits all' designation. Ploughing might be clased as a PDO on an ancient herb-rich meadow overlooking a sea cliff SSSI, for instance, whereas climbing on the cliff edge might not be. Generally speaking, there aren't many SSSI's where climbing is regarded as a problem, and when there are, considerable attempts are usually made by BMC or MCoS to compromise and accomodate both interests (eg bird bans).
Unless there is obvious and specific advice to the contrary (say, in the form of prominent signage) it shouldn't be a problem to climb within most SSSIs. It's the landowner who's normally more of a problem than a remote quango. When it comes to fears of prosecution, these are largely illusory, and more thoeretical than real. You would have to be knowingly and deliberately damaging the 'scientific interest' before action might be taken. Even then, agencies are often reluctant because of legal costs. A good (non-climbing) example is Scottish Natural Heritage's failure to prosecute a fossil thief who has systematically ransacked a SSSI quarry in Lanarkshire of its unusual fossil fish and flogged them to a museum in Berlin. They haven't even been able to get the specimens back from the Germans, let alone bring the villain to justice.