@BigAndos - Re: Customer service?
Unfortunately, their customer service is a pain in the backside. But unfortunately, that goes for pretty much all carriers in the UK that I have used over the years...
O2 - terrible
T-Mobile/EE - abysmal
Three - outrageously incompetent and annoying (especially once you move on and want to keep your number); spotty network coverage in the City; hidden limits for calls to other countries, which can cut you off in the middle of the conversation and will then block all calls unless you log into their site and make a payment outside of your normal bill cycle...
I ended up with Giffgaff now... It's using the O2 network, so you get decent coverage. They don't provide customer service themselves, and you don't pay for it. Sounded like a reasonable option to me, because you don't get customer service elsewhere either, apparently.
Their community based approach works rather well. Number transfer took less than 48 hours (apart from calls coming from Three, because Three kept routing my number internally for a week longer - straight to a voicemail I could no longer access; but that was Three's fault, not Giffgaff's)