Hi struan,
In an ideal world, HMRC will have issued everyone (PAYE) with the correct tax code. The tax code you have makes certain assumptions (like personal allowance - 2011 - 2012 that was £8105, hence tax code of 810L), along with other information gathered from previous years.
If your tax code was increased, it will have been based on a previous years' claimable expenses and HMRC are then incorrectly assuming that you will have those very same expenses every year. As I said, in an ideal world, this would be true, but as the first post in this thread explained, for most workers, tax claims and expenses will change year on. That's the reason we do our best to get the codes changed back to basic, so that the correct tax claims and payments can be made.
If you are happy that the tax code you have truly reflects the tax you should be paying, then you need do nothing.
If you are registered for self assessment tax returns (remember, you don't have to be self employed or CIS to be self assessed) then even if you didn't have your tax code changed and you feel your expenses have been correctly reflected, you still need to do a tax return for zero amount... and get off self assessment for the future.
The problem with them changing your tax code on previous information is that if your expenses are not as much this time, you will owe tax because you will have under paid.
Hmmm, not sure I am explaining myself very well!
If you're a client and you should have done a return, call the office asap to get it sorted. If you're not a client and you just want this explained a little clearer, call the office anyway - free advice and all that!
Hope this helps!!