The P60's for the 2012-2013 tax year must be issued by the employer by 31st May 2013

You can write to HMRC at anytime if you believe your tax code is incorrect and/or that you have paid too much tax.
At this time of year (End-of-Year) I would suggest waiting until all the End of Year submissions have been made by your employer.
The Employer Annual Return P35 and all P14's (P60's) must be submitted to HMRC by 19th May 2013

So best to wait until you get your P60
You need to request that they calculate you tax liabilities and compare with their records.
Normally this is computed automatically and any money underpaid or overpaid is notified to you by letter.

At this time of year NEVER respond to text messages or e-mails claiming to be from HMRC. It will be a scam.
HMRC will send only formal letter.