MarketPress sub-category displays all products instead

Hi. I’m not sure of why but if a visitor navigates to a product category page and selects a sub category, instead of seeing the expected content they see all products on the site. e.g. http://www.designasign.eu/store/a4-prints/ provides a drop down of A4 Print categories. Clicking an entry, e.g. Vintage Circus Posters, takes them to: http://www.designasign.eu/?product_category=157 which I think is a malformed address (although the correct category number). It displays the same content as clicking http://www.designasign.eu/store/products/ displays. Not sure if it was the recent upgrade that caused this? Have refreshed Permalinks, no change. Interesting, temporarily changing to your Grid Theme from the current Products Theme doesn't exhibit this problem; also I have deleted the Products theme from within WordPress and re-installed the latest Products theme from within WordPress, but the issue persists. My apologies if this is a Theme issue - clearly I don't know yet. Hope you can help. Thanks, Martin