hmm interesting, at least you know your router firewall is doing what its supposed to.
Just to rule out any false positives, do you use any of the BBC media services, such as iplayer etc.
The other thing that strikes a chord as a possibility certain BBC investagative shows seem to like to run experiments to find out how many PCs are infected and run random port scans. I think over the past few years theres been several programs such as watchdog etc that have taken such approaches in the name of investagative journalism.
