Hi, I am posting this despite finding the solution already as it may be of use to others.
I was unable to get the thumbnails to show for each product listed on the front page of either simplemarket or framemarket (parent or child).
I had been selecting various settings for hours including the 'Show Product Thumbnail' checkbox in the store settings.
I have now discovered that I needed to select a 'featured image' in the product details page for this to work correctly.
Bingo - thumbnails on the front page.
Thanks - hope this helps someone.
Please let me know if this is an incorrect procedure as I have had several install problems recently.