Sorry Phil, I remember seeing this and neglected to reply.
Bottom line is - No, I failed to source some sensibly priced plates.
My theory is that 28mm plates will do for both (25 and 28mm) box types.
The box on the Flashes that really got attacked last year looked typical of a squirrel attempt to me - the wood around the hole showed evidence of having been gnawed and stripped IMHO. Whatever the case, we need something and it would be best sooner rather than later.
The best I can find are these people:
http://www.thepetexpress.co.uk/birds-living-spaces/wild-bird-nesting/cj-nest-box-plates-28mm?gclid=CJqqp4nA5bYCFWXJtAod_F4A0g who, if 10 plates are ordered only add £3.99 p&p.
£20.90 = £3.99 = £24.89.