Substitute Moussa Dembele scored a sensational second-half hat-trick as Celtic extended their unbeaten domestic run to 29 games with a 5-2 win at St Johnstone. Midfielder Liam Henderson curled in the opener for the runaway Ladbrokes Premiership leaders in the sixth minute but Saints defender Keith Watson equalised with a header before an own goal by Hoops defender Dedryck Boyata had the home side ahead at the interval.