Same problem for us. All tests say ok. Too difficult to find a phone number but eventually did. I called (Removed)
Their first solution was to send an engineer out on 18th September, as if going 8 days without internet is acceptable. I said it was not and so they're sending a new router out. But I don't think this will solve the issue.
Where in UK are you?
Moderator note: Removed phone number
