Monies deducted from my Self Assessment refund - why?

I recently completed my Self Assessment and am due a refund. Within the return it asked if monies were owned in a previous return, and stated an amount. I noted that this was not correct. However when I have received my refund this amount has been deducted. How can I find out what this amount relates to, and how do I also get it refunded?

Hi Thomas Cresswell,
Unfortunatly we do not have access to induividual's records on the forum.
For queries relating to your personal tax situation please see Income Tax: general enquiries 
Thank you.