Chaps. Don't worry. It's a fairly basic clause in all third party liability policies.
Without going into the intracacies of policies of first and last resort, what this is is a non contribution clause.
In basic terms this means that if you claim on a policy and there is dual insurance in force, your insurers may ask for a contribution from the other insurers involved. In practice this does not happen very often.
My day job sees me insuring most of the big equestrian organisations in the UK as well as many game and clay shoots and this is common practice.