VM don't use fibre optic cable anywhere near your house.
I'm not sure what the white box is that the VM cable came in from. In my setup, there's one of the ntl: things on the right of this picture:
http://farm2.static.flickr.com/1153/...1a54979052.jpg
If that's what he connected to, then the cable running from it is probably very low quality and will impact your connection. Also it's probably unlawful in one way or another for one operator to go tampering with another operator's kit, and puts BT on shaky legal ground when there's a fault, since they're legally responsible for everything up to the demarcation point yet they've incorporated VM owned equipment into it.