HMRC has just announce they are starting their reconcialtion process for the PAYE system for the 2014/15 tax year, and expect to be completed by October. PAYE (Pay As You Earn) tax is that which is collected through the payroll, and is effected by the tax code.
There were issues last year but HMRC seems confident that the vast majority of PAYE tax payers will be correct and no action will need to be taken. If tax was overpaid a P800 notice will be issued, followed by a cheque within 2 weeks. If tax was underpaid then P800 notices will still be issued but you will be offered a few options which will also depend on how much is owed.
There is further guidance on what happens if a P800 notice is issued here, but as HMRC have said, very few people will receive them!