I still think Insurance is not the way forward, and a lot of good advice here, but as Treasurer of our society we have to have insurance, public liability, and both the National and Nottingham Associations have similar schemes which allow you to insure sheds, it works out at about £7.50 per £100 of contents pa. Your site officers should be able to advise you further. One word of warning though, if you join mid year you often have to pay an alteration fee (rip off) of about £30, just to change the paperwork!
;D