I think the USC and PAYE are currently treated separately by the Revenue.
I requested a P21 balancing statement a few weeks ago, but this did not review my USC.
I knew that I had overpaid my USC last year. I phoned the Revenue last week and it only took a few seconds for the guy I was speaking to to establish that I was due the USC refund.
Payment was made into my bank account a few days later.
In summary, if you think you are due a USC refund then contact the Revenue and ask them to check it for you.