Just an update on this for anyone else thinking of attending, I spoke to P&O ferries and they advised you only need inform them at time of booking that you will be carrying your air rifles and they will send you all the necessary paperwork to fill out and send back.
P&O Will then send you a letter that you show at customs and check in to get your air rifles through and once on board you give your car keys to staff of the boat.
Simples!!!!