Well, I put the purchase off because the car was laid up for winter, but bought the new Bosch S5 battery today and the trade card discount for battery and terminal converters was....
£0.00.
I did get £3.40 off a Haynes manual, though, and it had already been reduced by £4.00. I'm so lucky...
By the way, although the new battery fits the tray (just), the terminal converters are a pain, the terminal covers can't go back on, and my Ford battery cover doesn't fit any more!