Hi skeccy, maybe it can help in your case, check Fix cover by selected combination for ps 1.7.7.x
https://github.com/prestarocket-agence/classic-rocket/pull/229/files
mychildtheme/templates/catalog/_partials/product-cover-thumbnails.tpl
replace all
$product.cover
to
$product.default_image
This fix is for Classic-Rocket theme, but it should work on the classic theme.
Has anyone tested Showing thumbnails for all combinations on ps 1.7.7.x ?