Yes it probably is, they're generally pretty good at working it out but ask for a break down. It does say in the maternity leave policy at the NHS I work for that if you decide not to go back then you will end up owing them maternity pay.
You need to dig out your policy and read through it properly. Off the top of my head it's something like 1-8 weeks = normal (give or take) pay then 9-20 weeks = half SMP/ half pay and then the next three months (ish) at just SMP with no pay. The last three months are unpaid. If you took annual leave prior to going on maternity leave then you may also owe for that as you accrue leave, therefore in December 2013, you had not accrued all your leave for 2013 so you may not have been entitled to all the leave you took now that you have decided to leave.
It's complicated but your manager can probably help break it down for you. In my opinion, it's always worth going back to work for three months when your maternity leave is up so you don't run in to these problems. 3 Months isn't very much time at all.