Michael, online streaming services and regular web pages use different protocols and port numbers so that may explain your differing performance results.
Although unlikely to be the culprit, it's possible your computers are using a proxy server so check your public IP address and make sure it's a Virgin Media one. Post back if you need help with this.
A likely cause of these issues is your router so see if you can borrow one from a friend, colleague, or neighbour for a while for testing purposes. Also, because your problems are much worse in the evening then it could also be an ISP problem so you may want to get in touch with Virgin.