Thankyou very much for all the replies chaps, I feel slightly better about things even though I'm still mad-as-hell with myself.
Just to clarify, the demonstration was a running t/o, toe of the left skid dug in, heli started to lift on the rear of the skids so I dropped the collective and the following bump caused the damage. On reflection, the damage appears to have been caused more by the fact I didn't get enough right pedal in as the crease is on the right of the boom rather than the underside.
Tough lesson to learn. Engineers reckon new tailboom so it will exceed 4k and is deffo an ensurance job. Going to stump up the excess as a goodwill gesture and try to move on and learn the lesson.
Thanks again for support/wisdom
T