I've had exactly the same problem (same router) prior to this it had worked fine for years.
In my case, it turned out that the Router could not obtain the DNS Servers automatically from Virgin. So I changed the Router settings to obtaim OpenDNS Servers (
see here) and everything works perfectly now after power downs.
I have no idea why this suddenly happened .... I suspect Virgin's DNS Servers may have issues.
Hope this helps