Along with many others I recommend trousers with built in knee pads. I bought a pair from Screwfix and was so impressed a bought another one :-) I've had a look and I can't find the ones I bought (though it was a while back) but Screwfix do several ranges, and the knee pads to go with them. I wear them for all work because you can bang your knees in many ways without kneeling down and (touch wood) I have had no knee problems while I have been using them. You tent to forget that you have knee pads in until your knee contacts something, then you breathe a sigh of releif.
On mine, the front of the pockets is starting to wear out on both pairs, but I think I will patch them because the rest of the trouser is fine. Although they do have self-unzipping flies which can be draughty at times.
Cheers
Dave R