Finally!
At random I decided to leave a blank password in the login details and clicked twice (ignoring the prompt to enter a password) and I managed to login.
I only wanted Mulsanne camping, so there was no issue with selling out, there were still Blue Nord/Sud, Houx, Houx Annex etc still showing available.
The allocation available looks much better than last year's Beausejour or bust selection - just a shame they hadn't explained the login system very well....