You can get a new lead for a few pounds from Wilko, B&Q etc. At the same time buy a 4 way chocolate block connector- like this.

Chop off the end of your present lead, and chop the new lead in half. Strip out a short length of lead in each one (caution: you didn't say if the BT end was upstairs or downstairs... make sure it's not connected to line - 90V at 25Hz feels awful if someone calls while you're holding the end!) The colours should match up, so just connect them via the choc block.

If the colours don't match up, first panic then post again and we'll go from there.