Betty,

It sounds like the moth balls will do the trick plus possibly keep them from coming back. I once knew a farmer that had problems with groundhogs building their underground dens on the hillsides on his farm which made running a tractor and equipment dangerous because as the upper wheel on the tractor went over the den entrance it could cause the tractor to overturn. Many farmers in the area came close to being killed this way and some were killed. This farmer would locate the den and take a fire proof hose of some sort and stick one end down the den entrance and put dirt around it and then take the other end and put over the exhaust pipe on the tractor. He would then start the tractor and fill the den with exhaust fumes, pull the hose out of the den and plug the entrance with more dirt. This worked for him. You might try the same thing using your lawnmower and flooding the cavity under your patio slab with exhaust fumes. I feel you need to do something soon because mice can multiply very quickly and really be a problem.

Just a thought.....