By jove, I think they've got it!
In order to force the situation we added a kink to our run wall; the feeder was surrounded on three sides. No more reaching in from the side now! This certainly changed the group dynamics and within 24 hours we spotted all four birds eating together - all stood on the platform. Frog quickly gathered courage after this change-over. Like Fizzy, she could no longer reach the food without standing on the platform and she was spotted using the feeder without any help just a couple of days after the move. Lemon, whether by design or accident, was also seen standing on the platform to eat (albeit with one foot still on the ground) at about the same time. We hadn't yet seen Pepper use the feeder properly, but she was eating heartily while the others were holding the platform still so we declared a success and removed the training bolts completely.
Fizzy, again, didn't seem phased by this at all (she does seem to have the brains - or maybe the brawn - of the flock). The platform sits at about her chest height now, but a run and a jump and up she goes. The others may take a little longer, but with Fizzy able to open the lid for them all we won't be having any hungry chickens yet. All in all, a successful training program.
What do you think we should get them to learn next?
Update: Another week or so passes and Fizzy is still the only one who can open it properly. Lemon sort of does so without realising she is at which point Frog or Pepper will see the food and pile in, inadvertently standing on the platform and holding it open. We may have been a little premature shutting it completely, but no ones going hungry yet so we have decided to persevere. Chickens aren't known for their huge brains after all...