You could try using the form ESC A19 to appeal - start your search here:
http://www.moneysavin...ily/2010/09/tax-chaos
You should be able to find a template - sorry I do not have it to hand.
Just bear in mind that you are not a 'person' to HMRC (Her Majesty's Royal C**k Up'), but only a reference number, so keep the tone of any letters objective.
It is their error, but their defence is always that it is up to the individual to check his/her tax code. Their mistake, your fault.
From my experience, they blindside you with jargon and obfuscation and pursue you relentlessly until you give in from sheer war weariness.
You can come to a payment arrangement with them, or they may threaten you with having to do self-assessment returns in future.
However, if your name is Goodman Sachs or Vodaphone you can skip merrily away.