I finally picked up a box of Shepherd Neame lastnight. Master brew, Bishop's Finger, Whitstable, and, I think, Spitfire.
Anyway, I opened the Master Brew, and straight away the difference was apparent. It's like a different drink when it has been hidden away from nasty light. A lot more flavoursome and drinkable.
Not a bad price either. I may have to pick up another box.