Buy 2 F type connectors (like the connectors that link the cable in to your box) Also by a back to back adaptor and some self amalgamating tape.
Put a connector on each end of the severed cable. Plug them together with the adaptor. Wrap the joint up with the tape to keep the water out.
You can get all the bits you need from Screwfix or Maplin.
Fuzzy
April 2009