I was just thinking about this recently, and discovered that I might be eligible for a refund as well.
I last had a paying job in 2006 (a short-term thing, related to my field of study). I didn't even think about taxes on that until my husband mentioned something about it the other night. According to my records, I paid about 500 pounds in tax, but since my total income in both countries for that year was something like 3,000 pounds (and I haven't had a job since!) I definitely should have had a refund.
Apparently the limitation on these things is 5 years and 10 months from the end of the fiscal year in which the overpayment was, so I've still got time to inquire about it. It sure would be nice to have that chunk of change--it would knock out one of my small debts, which would be fantastic!