Text below from an email sent from Virgin:

We are changing the way you choose your seat and are currently moving to the new process. You’ll be able to choose your seat, free of charge, using Manage my Booking but in Economy we will be charging for choosing seats in advance on selected flights from 1 April 2014. Seats will be free to choose at check in. Click http://www.virgin-atlantic.com/gb/en...ssignment.html to find out when you can assign your seat.

That's a bit annoying.