CSA said my case would be closed but I’ve returned from living abroad to find I owe a large figure

I phoned to inform the CSA in Dec 2011 that I was moving abroad in Feb 2012. They told me the case will more than likely be closed and the only way it could be re-opened is if I returned and if the Mother got in touch to say I was back, and opened a new case. They also said someone would phone back in Jan to confirm – I left the country in Feb and no-one had called.

I sent them a letter saying I’ve left my job, closed my account and am moving abroad – which I was. They never replied. I stupidly thought the case was closed, stuff didnt work out abroad and came home. They have now tracked me down and said I owe them, but haven’t told me how they have calculated it. The figure is something like what it would have been from then till now. Do you think I can get this disputed? Very stressed as they say they can take immediate action. Please advise!