Hi monkey!
Tax is not routinely refunded (under PAYE as opposed to self assessment), you have to ask for it - also, make sure you state you want to apply for the past 6 years as if you don't they might only look at the most recent tax year.
Most people do pay the right amount of PAYE, but it's worth checking if you've had a gap in employment or a period of reduced income.
But, a little word of warning! You may (by requesting a review of your tax affairs over previous years) find out that you receive a demand for underpaid tax, as well as tax owed back to you! Sometimes, these underpayments can go un-noticed by HMRC, some are written off without you knowing and can be brought back into payment! Any refund you're due may be reduced to pay back these underpayments.
I could offer more advice, but I'd need to know much more specifics of your previous employments, tax codes etc.
HTH x