i heard on the lunchtime news that a new web site is available so you can check yourself to see if you are entitled to a refund. I missed the address of the site.
You can also do this quite simply by writing to Revenue including details of any unclaimed expenses (e.g. pension contributions, medical expenses etc.) or suspected errors (e.g. in tax credits etc.) and copies of your P60s for the relevant periods and asking them to recheck your file.