While it is entirely possible that I am wrong, surely it's more a matter of mandible depth than length? Parrot actually has a slightly longer bill than Common.
It may do, but the depth will be proportionate to this (the length) and in parrot
tends to give the bill a 'square' appearance.
The male above reminds me of the dubious male bird that I had on my blog some time ago (deleted by hackers !) that quite a few people thought was Scottish, but in fact it was (biggish) Common - in the hand it's bill depth was 11.2mm and it gave Fc4 on release. The culmen was more downcurved than normal for a curvirostra. Photo below.
For me the Yorkie bird's culmen is not nearly downcurved enough for Parrot and is similar to the Deeside bird I refer too. It is worth remembering Dick Forsmans advice on Parrot Crossbill identification in the field - the bill can actually appear
small in proportion to the really large head ! I think you too have noted the importance of head size in crossers before Stuart ? I tend to do it on body size in flight - 5-7g is a big difference between Common and Parrot, with scotica more like Common.
To the photographer, if you were that close get some calls - then we will know for sure !
Lindsay