Found the error. It was related to the permalinks. For some reason a custom structure was marked and it was resulting in a // appearing before the portfolio post ending, i.e. …./portfolio//…. . Have selected post name for the permalink setting and all seems to work fine now. Unless anyone is aware why with the Virtue theme we should have a custom structure selected for other aspects of teh site to run properly (looks like all the shop items can still be added to the cart OK).