In reply to George Ormerod:
Declaring your climbing activities at application stage forms part of the disclosure rules. To quote one major insurance company's criteria:
"If you do not disclose all relevant information or you provide incorrect information this may result in the non-payment of a claim. If you are in any doubt as to the relevance of certain information please disclose it."
Whether the application form asks directly if you climb, or indirectly if you take part in an activity that carries an additional risk, non-disclosure could therefore invalidate a claim, even if the claim is not related to a climbing incident.
On a positive note, there is an element of 'Good Faith' that insurance companies generally adopt, in that once your contract is in force, it remains valid irrespective of changes to your activites. So if you were not a climber, and had no immediate intentions of starting when you took the policy out, a future claim, even as a result of a climbing incident, would normally be honoured.
There are some contracts whereby insurance companies need to be advised of a change in occupation or activity and so it is prudent to check with your existing insurer if you fall into this category.