The Flying Pram,
Well, I never ....
BT Shop - BT Plug in RFI Filter (1000215)
(don't know why you need a BT engineer to plug one of these in for you though !!

)
Howver, I too am left wondering how this would help.
I can't remember where I read it, but somewhere someone made a great comment that went something like this .....
"ADSL is delivered over copper, which on average consists of runs in hundreds of metres, lots of weird and wonderful stuff goes on along that cable route, and yet people still persist in worrying themselves sick and wasting money cabling up the last few metres in their property".
And I kinda agree. If there's going to be a problem at cable level, it's unlikely to be something in your property as long as you follow basic precautions !