If you leave within the first 3 months of returning to work you have to pay 100% of the top up, within the 3-6 months 75% etc, so basically you need to stay a year and a day after you have returned to work to not pay back the top up.
Our contracts were amended during the year, and we all had to sign the new version, however this was not advised on the contract we signed, can it still in enforced?
I assume the company asked you to sign a maternity contract before you went on maternity leave ? In which case if you break it (i.e. don't return after your maternity leave and don't pay back your top-up salary) then you're in breach of contract.
Technically they would have to sue you I suppose to get the money back and the question you would have to ask yourself is a) would they bother doing this b) would you want this hanging over your head either financially or morally !
Unfortunately maternity contracts are common place these days and while it seems like a good benefit it's actually an easy way for a company to keep you on the same salary for a couple of years at least. (Been there !)
I would advise you to ring either the citizens advice bureau or the equality authority - have a copy of the new contract you signed (if you didn't sign a maternity contract - this may have been a 1 page form before you left on maternity leave) to hand so they can see what's in it. If you signed it - then it's applicable.