Thanks all. I've tried some different insoles, prior to adding plastic etc, and they didn't help - will look at the ones mentioned.
Re. silicone sealant - interesting, I've been tempted to try epoxy putty, but will hold off on this.
Can anyone, I wonder, shed light on why Shimano, with all the tech & refinement that goes into cycling shoes, essentially leave an inadequately padded access hole in the very place where 'cleat forces' are concentrated. I've never had to replace a cleat fixing plate - perhaps some do. Maybe it's just that my thin (rather than narrow) feet are prone to ache with this kind of design.