Trust me, as someone who recently had success with getting BT to sort out connection speed issues, you want to be phoning them and harassing them to sort it out.
Do what they say when they take you through all the virus scan, checking your router, etc. and eventually they'll realise it's a problem in their network and fix it.
Do what they say when they take you through all the virus scan, checking your router, etc. and eventually they'll realise it's a problem in their network and fix it.