In reply to Robin Lincoln:
Stayed last year for around a month at the same time. The reservation system lasts until around mid-end of October (25th rings a bell) and then they close the old C4 and move everyone over to the new section where the showers are. After this it's a first come, first served system where you find a pitch, go to the kiosk and either have a ranger put in your details or fill out a paper form and pay via the National Park App (there's WiFi available by the kiosk). A lot of people had moved on by the end of the month and the whole place took on a quieter feel compared to our first few weeks.
I'd try and get it booked ASAP as we had to wait a few days when we arrived before getting a spot as they were all fully booked, despite having a spot from the Monday onwards. We didn't have a car so had to wing it with pitching late on an empty space and asking to use other folks bear boxes or the ones in the day parking, and had to leave all our stuff there too during the day while we were climbing, which was far from ideal. This only happened for a few days but worth being aware that you may not get a pitch straight off when you arrive unless it's pre booked.
The reservation bookings were fairly stress free after I found out there was an app, although using the Parks website wasn't too bad either. The rangers at the kiosk were friendly and helpful.
Post edited at 18:23
