simple enough job, from memory i think there is two screws near the top of the clock surround and one near the ignition, disconnect wire from switch near ignition, the one that dims the instrument panel, remove clock surround.
couple of more screws holding the clock in. remove these. pull the clocks out a small bit so u can get your hand around the back to unplug the wires, now clocks are free to remove and bobs your uncle.
don't know if halfords do the bulbs or not but they should be around £3 or £4 of a lexus main dealer.