I think mat pay is based off your average earnings from week 16-25 of your pregnancy so if you're already on mat leave in September you won't get the pay rise until you return
I was on maternity leave when the last payrise came into effect, I got a lump sum in my November pay which I wasn’t expecting. Still a nice bonus even after the tax
I think if you have any enhanced mat pay after September then it should increase (or you'll get it back pay whenever it's been sorted for everyone). If you're doing shared parental leave then your 'return to work' periods after September should benefit too. Unless they say the pay rise is in effect from earlier than September, which I doubt.
I’m due to go on maternity leave in September. I asked our HR if any eligible may pay would be backdated and they said it would. Think it’s from September - we normally have our back pay in Nov pay packet. You should have the 5.5% rise regardless of being on maternity leave!