Your maternity leave is based on your past employment, not the promise of future employment.
If you've worked the minumum amount, you will be legally entitled to your 10 weeks of partially paid leave. What you do after that is your own business so long as you keep to your contract. (So, if you have to give 30 days notice to quit, that still applies).
You do not have to pay anything back so long as you follow the letter of your contract.
That's the legal answer.
The common sense answer is that, out of common decency -- and to keep up good relations in your industry -- you should probably let your boss know if you are not returning.
That does not negate your maternity benefits -- they are still your legal right.
However, it's not a very considerate thing to keep your company holding out for your return -- and probably holding a job / position for you -- and then dumping them at the last minute, when you knew all along you'd be leaving.
Much of this has to do with your relationship with your company.
I've been with my company many years and get along with my boss. Both times we've sat down and had frank discussions on the fact that I wanted extended maternity leave, and that I didn't want to return to very late or long hours with young children at home. So we worked something out honestly.
If I knew I wanted to become a full-time stay-at-home Mom, I'd give him a respectful amount of notice.