You only have to give four weeks notice (28 days) if you want to change mat leave dates. So the earliest you can leave is four weeks from when you tell them of the change, but you can't start before 29 weeks (although that week starts on a Sunday even if you aren't due until the Thursday like me - mat1b runs from the Sunday).
My last day is 29th September, four weeks leave after that so mat leave at 34 weeks officially. Hate, hate working there so can't leave soon enough - four weeks to go! Yippee!