Raggytash is right, the PAT test itself is not a legal requirement. However it is a recognised means of proving that the equipment has been checked. A band going into a venue to play will have to comply with that organisation's health and safety policy, which will usually require evidence of a PAT test (and sometimes risk assessments and work method statements too). Whether or not these are legal requirements is beside the point - the venue is entitled to set its own safety standards, and I can guarantee you won't get anywhere trying to argue the toss with some jobsworth (who prbably won't have the authority to deviate from the policy) half-an-hour before the gig is supposed to start. Getting the equipment PAT tested is fairly cheap, ticks the appropriate boxes and everyone is happy,