Maternity pay and going back up to 5 days

After going back to work following the birth of my daughter, I went down to a 4 day a week contract. I’m now debating going back up to 5, partly for career reasons, party for financial reasons. My husband and I are planning on trying for a second baby in 6 months or so - does anyone know how long I’d have to be back at work full time to qualify for full pay whilst on mat leave? The difference in pay would nearly cover my LOs nursery fees each month, so ideally would like to make sure I am eligible for the full time pay!