The pump is actually quite close to where the cold feed tee's into the heating system. So I've taken the pump off - as expected there's no water coming through from the cold feed. Feeding a wire up the pipe (where the pump was) there's clearly quite a hard block at the t juntion where the feed joins the heating system. I cant unblock it with a wire coat hanger so I'll give cutting it a go tommorrow.... thanks for the help.
