The leader may have had a nick in it because it had been snagged a couple of times during the morning - sorry, I binned it and re-rigged. I didn't check as I was busy looking after the other needs of my guests like food, drink (non alcoholic) and bait.
When I shut off the drag something had to break and there were two of us hanging on to the rod!
Big Al (<>< has suggested I should have taken the drag off completely and the fish would have stopped pulling. Never considered this as I had so little time to react. I may have tried to chase it if it stopped for a little while and showed itself (it could have been an angry old shark) but when your not prepared, you make mistakes big time.