Are you able to call at all today? I'm around until 4pm...
That is almost certainly a scam, not sure how they got your details, but Teamviewer will allow access to all of your personal information on your computer, and can insert trojan software and viruses.
You should never allow access to your computer by a stranger.
What phone number did they call from?
They gave the number 020 3289 0864 (gave names David & Jenny) but call list shows 01391 735842
BT only call from the normal 0800 number, so I am afraid it was a scammer
Lets hope they did not extract too much personal information, especially from banking sites.
Good idea to keep a watch on any transactions.
"We will never ask customers for personal information out of the blue and we’ll never call from an ‘unknown’ number. If we’re getting in touch about your bill, it will usually be from either 0800 328 9393 or 0800 028 5085. "
From the last page I linked to.
Customers can link directly to the Action Fraud portal from this website or call Action Fraud on 0300 123 2040.