Not sure that it beggars belief.
On the one hand there can be a perfectly legitimate belief that the money is better in your account than in someone else's, on the other cashflow may dictate paying on the 19th of the month rather than the 6th. For some people, £300 isn't a wholly insignificant sum, especially a couple of weeks after Xmas spending has finished.
As for 'strong reminders,' have these been emailed to those who registered on the 6th and haven't yet paid? I've not seen anything other than posts here and on facebook - I don't think that's a reliable method of reaching everyone.