When are nursery expecting payments to be made, I'm confused!

[Hmmmmminteresting]

Ds is about to move nurseries as of the 8th sept, the nursery advised that payments are due on the 1st of each month but will be classed as "late" after the 9th.
I assumed this meant the whole of septembers fees would he due as of 1st September, which was fine.
However they've now emailed me a payment reminder for being late with my first payment and attached septembers £800 invoice to it?
Now im stressing that setpember was due in advance and I'll get another bill through due on 1st September!
I cant get my head around it. Am I right in thinking that at the 1st September I should only be paying my september bill?
I've called but the admin lady has left and isn't in till weds and apparently nobody else there knows the answer

Do you have to pay a deposit for the space? Is that what they could be chasing. I don’t see how you can be late paying for Sept when it’s only 17th August. Just be a wear if paying via tax free or childcare vouchers these take longer when there is a bank holiday at the end of the month.

All they asked for was £50 registration which I have paid! I'm so confused. No mention of deposit anywhere I have reread the ts and cs this afternoon.
Yes I spoke to her at the start about the tax free childcare as I usually pay our current nursery around the 3rd of the month. She said this will be absolutely fine :-) thank you