Send any P60S or P45s that you think you might be owed a refund on. Send these with a covering letter to your tax office stating that you wish them to check the tax liability. They can go back six years. If they do not have a complete job record for you this may hold up the process but they will send out a P91 form for you to complete once the record is complete i e any periods of employment or unemployment then any refund due will be issued.It may speed up the process if you go to the HMRC website and download a p91and send it in with your claim.