PAYE incorrect since year 2000

Morning all.

A friend in his 60's has received a notice saying he has received his personal allowance for both his pension and small employment income since 1999-2000.  HMRC are obviously stating that he had his allowances twice as opposed to the BR code being run on his payroll and will shortly be issueing a statement of what they believe is due.

Initially how far can HMRC go back to reclaim this?  I always had 6 years in my head but remember reading something recently about new powers being given to reclaim tax much further back if HMRC claimed it was careless or on purpose.

Help greatly appreciated.

Cheers!