Just to point out, that with a P21 balancing statement, it could also show that you owe them money. If you did not pay the proper tax for some of the years you were working then they would have to reclaim this. Very rarely happens but I do know 2 people it happened to.
On the other hand, the refunds are good - hard to estimate what you may be due without knowing what you were paid for each year and what tax you paid.
Also, not sure if you considered this, but you can also back claim for rent relief - another good source that some people dont think of.