Tom, I may be able to help on the insurance side if you pm me your email. I don't see that the nature of a fatass type event causes major problems - the concept is not dissimilar to a group of friends meeting up for a run. Taking payment would complicate things because you create contractual issues, and so would someone who is completely out of their depth. The last TP100 had some serious weather problems and several people ended up in hospital but I'm not aware of any claims. Life always involves some risk....