No promises here, but I stumbled upon a site where you tick/click a box with any and every allergy you have including gluten on the type of grocery item you want. Click the bright green find it button and hey presto a printable list of all of the products in all of the supermarkets appears.
A little warning here: please double check when you go to the supermarket just to make sure that the item hasn't changed any of the ingredients since it was listed. Overall, I thought this would be a massive help to everyone - especially as it applies not just to main supermarkets but also to many of the smaller ones.
Here is the link: