I've been trying to get to the bottom of this recently because, for the first time ever, a venue at which we played - a barn at a farm - insisted our equipment was PAT tested. We've just bought a PA from a fairly well known band, one of whose fathers posts on here, who had had it tested in 2008. The school at which I work has all the equipment tested annually but I do not know if this is a legal requirement or just LEA (or whatever we now call it) policy.
In the time I spent searching I was unable to find a definitive answer. The most common opinions where that it was completely optional, it had to be done annually, it had to be done every three years and as long as you've had it done once it's ok. Eventually I got bored and looked for an electrician on the Sheffield Forum. Found one ... did it the next day at my house ... 25 quid.
Sorry that none of that helps but, to get to the crux of the matter, if a venue insists your stuff is tested then you'll have to have it done.
Annoyingly, since PAT testing is simply a confirmation that the Earth connection works, anone with a multitester and a screwdriver could do it for themself. Sadly - the world don't work that way.