Morning! I had this too this morning - green wifi light flashing and slow speeds... I went in and turned off WPS and rebooted and that fixed the flashing light.. but wifi was still slowed - checked the settings and it seems that the router had changed the channels I had set up, even though I set them manually and turned of optimisation!!
Well, tried to change them back, but it kept changing them back, but with a bit of perserverance and swearing I finally got them to stick and it is now working as normal.
I have now managed to get the light to go out after turning off WPS and rebooting. However, if I then do the reverse, it starts flashing again. It's about time Virgin sorted this sort of thin out as, with WPS available, it presents a real security risk to our system.