Sounds massivley dodgey to me mate I would personally be quite concerned, sound like there after your motor to me...
Yeah that would be my first thought.. did you do a 1471?
If you felt the call was threatening you can report it to the police who can go to BT, who will provide (to the police) line traces showing the original calling number, and what company/person that phone number is registered to.
I would probably do it just in case.
