Spudsey, the situation you describe throws up an interesting anomaly. The regulations apply to the use of electrical equipment at work, so if you are performing on a truly amateur, unpaid basis then it could be argued that you are not "at work" and therefore don't need PAT tests. I'm not a lawyer, and you'd have to get a qualified opinion on that.

Nevertheless, a venue may still want to know that electrical equipment brought onto their premises is safe, so you may well be asked for it. From the venue's point of view, the safety issue exists regardless of whether the event is for a "good cause". However, as you've found, there are plenty of venues which take a more relaxed view.