Don't do that David. There's nothing wrong with mossy sax's. Go with what Carlo says, and even if you leave them as is, you'll find there will be new autumn growth which will fill them out, quite soon. Each tatty rosette will make new little ones and so fill up the spaces. But the gravel around and under is good. You should also take a few rosettes off, as cuttings, just putting the stems into damp sand, and they'll root in no time, as new, chunky little plants. Put something into the tatty bits and round about that will do for grass grub and vine weevils. Both LOOOVE all kinds of Saxifrage.