oh yes, have seen those I think. I wonder when they start doing neck sizes for shirts. I guess it must be once they are big enough to fit into a man's size!
If you have no luck with M&S or the supermarkets, perhaps try one of the uniform outfitters (you know, those places that do the private schools) - quite often they have a big range for different sizes and shapes.
I find that traditional uniform shops stock by collar sizes - both of my children needed 11.5 shirts for reception due to the top button. The issue seems to come around size 12.5 which is a fairly typical collar size for 6 thorugh to 10 year olds, and then you need to find places which have different collar/chest sizes. M&S shrts for the 7 -10 age group all have 12.5 collar size but different shirt sizes. John Lewis/Peter Jones will also sell by collar sizes and they will possibly have the greatest selection of shirts ime. All of these shops will be staffed mainly be students at present so you might want to look out for/ask for a more seasoned hand.