An employer is not allowed to discriminate an employee for being on maternity leave in any way, so if you decide to not return to work, then simply give your employer notice as required in your employment contract.
You will continue to build up holiday entitlement until your maternity leave or notice period ends (whichever is sooner).
You won’t have to pay back any statutory maternity pay you have received, but if you have been paid any enhanced benefits from your employer, you may have to pay these back if you don’t return.
To clarify, at the time of writing (December 2014), statutory maternity pay is either £138.18 per week or 90% of your normal average weekly pay (whichever is the lower). Some employers these days may offer a contractual maternity pay that is more than the statutory figure, so if you give notice that you won’t be returning to work during your maternity leave it’s possible that you may have to repay the extra contractual money, but not the statutory amount (and even then only if you’d previously agreed to repay this in the event of you not returning to work).
The legislation surrounding maternity leave and maternity pay can seem complex, so if you decide to seek legal advice then please remember that QualitySolicitors provide Free Initial Assessment. This is a free short telephone chat that enables you to tell us all about your situation, and also allows us to explain how we could help you if you decided to get help from one of our expert solicitors.