I managed to fix this by amending the code on line 103
$('.woocommerce-main-image img').attr('src', $src.replace('-<?php echo $product_thumbnail_size; ?>','-<?php echo $product_single_image_size; ?>'));
I removed -<?php echo $product_single_image_size; ?> from the line so it just reads as ” now. This means that the main image file isn’t looking for a specific size, it now just looks for the main image src.
Edit : my line 103 now reads as $('.woocommerce-main-image img').attr('src', $src.replace('-<?php echo $product_thumbnail_size; ?>',''));
@adamkinder – Your solution worked, thank you! I hate changing a core plugin’s code, so hopefully this is corrected on the next update.