zanussi washer dryer ZWD16270w1 won't dry properly?

I have had this washer dryer about 18 months. Recently it has stopped drying properly, normal loads take 3 or 4 times as long to dry as they used to if they dry at all, even if I run an extra Drain cycle first. There are no error messages displayed or unusual noises, the dryer heats up and the drum rotates normally, but the drying cycle sometimes stops about an hour before the programmed duration. This happens on both the Cotton and Synthetic drying options. I have checked the filter (which wasn't blocked anyway) and cleaned the machine with proprietary cleaner.