We have ours through NSALG. As a Society we are affiliated - that costs £2 a head - and then we have insurance through a company called Newspace. This gives us Public Liability and also Employer's Liability, which covers us not just for anyone we pay to do jobs, but for any communal jobs or working parties. The will also insure any communal buildings, but members' sheds have to be insured separately by the individual concerned.
As a bonus, affiliation to NSALG also gives you access to legal advice and they run a cut-price seed scheme with King's Seeds.