I’ve dealt with them twice – or rather the approved arbitration service.
You need to come to a deadlock position with your provider first, so fire off a letter with your complaint (signed for). Give them time to respond and then fill out all the stuff from the arbitration service.
First time I got £250 compensation and a refund, second time nothing – and yet my second complaint was far more serious than the first, so it is a bit random.
Both complaints were about the telcos failure to refund me for a service they did not provide. (No internet for a week or so and lying to me).
Marko