Just need to rant... maybe someone can relate...!
I started a new part-time job in July & have not been able to get HMRC to assign me an appropriate tax code.
It doesn't matter how many well-written letters I send (even with tables in them!!), or if I update my earnings on the Govt Gateway.
First they had me on a code that gave me only £400 tax-free, now today I get an email saying they've updated my account (yay!), so I log in all excited, only to see they've changed the code to one that equates to £600 tax-free.
I started crying in frustration! I only earn £16k and should not be paying thousands of pounds in tax each year. 
There are never any webchat advisers available and talking to someone on the phone also seems impossible. It's driving me mad! What do I do??