Your seat your choice.
Thanks for the speedy reply. I've now booked my Cancun trip, safe in the knowledge that we stand a good chance of getting the seats we want, if we logon when the calender opens at 98 days before departure.
The funny thing is though, all the premier cabin's seats are blue (unavailable) as are a few economy seats but the majority of economy seats are white (available) and can be 'picked up' even though the flight's not until October!