Can you replace just the cord in the light pulls and if so is it easy to do? and how do you do it? or is there anything you can buy to clean the cord which feels of cotton rather than nylon?
Yes yoiu can - some pull cords have to be changed from inside the switch housing (you have to take the cover off so isolate the power first) and others have a screw fitting just below the switch (much easier).
A DIY store or outdoor shop will sell you replacement cord - take a sample with you. And you can buy a new gizmo to go on the end too.
It's a very easy job but you'll have to stand on a chair.