Dave-T

i allways use a foam mat. I find knee pads cut into the backs of my legs and slip down my legs. The mat i use is a rubi one, and its proper comfy. I got mine from millcott tools, which most of you wont be able to buy from, as its not for retail customers, but they should have them in decent tiling shops. Failing that most diy places will probably have cheaper gardening ones.
----
another thing to bear in mind actually with knee pads, is if your laying a victorian style floor, then dont use the knee pads with the plastic caps on them, as they scratch the tiles. Learnt that one the hard way!