I work for one of the major banks and over the last couple of years have sent tens of thousands of customers refunds by cheque (closed accounts) and many thousands remain uncashed over a long period. Now a refund for a few pence you could argue is not worth cashing, but a good proportion is for hundreds to thousands of pounds. Using a tracing company we found 60% were still at the address on the system so have no doubt received the cheque and ignored it. Maybe some people don't believe they are entitled to free cash, thinking there is a catch even though there was a letter included explaining why.