In reply to Rog Wilko:
For the last few years I've used one of these, on and off:
http://shoft.co.uk/
Not a back support as such. It keep the lap portion of the seatbelt where it should be, over the hips, helping to prevent slouching. It's not a complete lock, and it does leave the upper seatbelt totally free to move, so from a practical point of view it doesn't hinder anything. I swear by it, especially for long drives. I find I need to re-tighten it by wiggling back in my seat and pulling up on the shoulder portion every hour or so.