I use brass screws and lead flashing for washers. Make a hole in a small piece of flashing. Put the screw through it. Screw the pan down (not too tight!). Use a knife to trim the lead sheet. You end up with a flush screw head and "washer" that is an unobtrusive fixing. To fix the basin, locate the pedestal and basin first. Screw down the pedestal, not quite tight. Put 3 generous dobs of silicone sealant on the back of the basin, and a good bead around the top of the pedestal. Offer up the basin, hold it on the wall while allowing the pedestal to take the weight. Screw on the basin (helper to hold). Leave for silicone to cure, then trim around basin base/pedestal with single-edged razor blade. The silicone (oh - of appropriate colour!) at the back will not show unless you've put on far too much. Finish fixing, plumb in.