I also think you can start maternity leave whenever you like, in our maternity Leave policy at work it says we must give 10wks notice of our intended date for maternity leave, but ofcourse if we had to leave earlier due to medical advice etc we could finish earlier with a Dr's note! But I could have given 10wks notice from the date I had concieved (IYKWIM)...

Also in our policy we have to write a stat dec saying we will not undertake any other paid emplyment whilst on maternity leave! So I cannot do any part time work whilst on mat leave! You should probably check both comapnies policies!???