Portable appliance testing is one of those fun things that always seems to come up now, along with PLI of course.... It's unfortunately a feature in our modern society to protect the polar bears and the ozone...
Anyway, it's not a legal requirement, the IE rules are still hazy, and in general as may have said it's worth doing just to cover yourselves and show a degree of responsibility and professionalism to the client and venue. I'd say one in 3 tests I find faulty cables with no earth, or badly wired extension leads (wrong polarity, or just screws loose). Most gear passes, with only the oldest of equipment failing, usually on a bad earth, which is often fixed with a quick clean, and re-connect.
I charge around £60 if you come to me, or £80 for a visit within an hour from me for a typical full band set up of up to 50 items, Less for solists, DJ's etc.. with less gear.
Regarding just doing it yourself, I've once had a venue contact me, as my details are on all my tests, asking for a certificate of competency and certificate of calibration for my machine, both of which I have... If I hadn't, the band may have found themselves losing the gig !! Yes, you could DIY it, but you'd have you buy the equipment, typical half decent PAT machine is around £500, plus you need to get it calibrated ideally once a year (£80 a pop), and go do a course to show competency.