I've found the critical thing to look out for with strap-on knee pads is the material and positioning of the actual strap.
It has to be a single stretchy strap and postioned at the bottom rear end of the pad (where it can sit on top of the calf instead of bunching at the back of the knee), other than that I don't care what they're made from but I quite like these atm, think they are made by Nailers.
[DLMURL="http://www.wickes.co.uk/Wickes-Heavy-Duty-Knee-Pads-Black/p/186922"]Wickes Heavy Duty Knee Pads Black | Wickes.co.uk[/DLMURL]