Tumble Dryer Drum not turning?

I have a White KNight Sensordry tumble dryer, when I start the dryer the timer makes a sound as if it is straining. I have removed the top and when I move the drum with my hand then it turns ok, but when it comes to reverse it stops again, move it with my hand, and it starts again. What part would I need to buy? or any ideas why the drum doesn't seem to have the power to turn until i push it?
Thanks