Whirlpool Tumble Drier AWZ 2650 Reduced drying performance.

Machine is taking much longer time than previously to dry washing.
Filters thoroughly cleaned in usual way, and continuity checks made on the two thermostats and heater. All appear ok although one heater connecting terminal seems to have over cooked at some time and this was cleaned just to be sure.

I’ve run out of ideas so any advice would be much appreciated..

WJ.