I had this a few years ago. Get a mole trap, one of the scissor ones. Find the run, cut out a section of the turf put in the trap. Cover trap with dirt, flatten all the raised areas and wait until you see the trap has worked. Job done. Its amazing how small the hole is they go through and it's perfectly round.
A couple of days before I flattened all the raised areas to get a feel for the run it would keep on using.