I mean it’s a total disgrace really but these are the terms:

The full balance will be required four weeks before departure for short haul bookings* and seven weeks before departure for all other holiday bookings. *Selected European ski destination require the full balance five weeks before departure. Please see Manage my Booking for the exact date applicable to your booking.

The problem, of course, is that you would probably be able to fight it in court because they have sent you a receipt or advertised on MMB the incorrect time for full payment but by the time you do that no doubt the holiday will be gone and you will find it very difficult to get them to honour things at whatever price you had originally paid, let alone double tier points or anything else you might be eligible for.