I had tried electronic sonic devices, electronic mouse boxes and every poison on the market, with limited success. I then tried Rentokil's Fast Action Mouse Killer and did it the trick for me. To quote their blurb 'Fast Action Mouse Killer is a unique, highly palatable and effective rodenticide product, which kills mice within 24hours. Sufficient bait to kill up to 100 mice. Bait is kept securely in pre-baited boxes making it child and pet friendly. Contains Alphachloralose.' They sell it at Plough Homecraft. Good luck!