Thanks for Wol-arp-mon, it's wonderful!
Some niggles for me over the interface and the way it does or reports the scan but otherwise exactly what I was looking for to remotely wake up or switch on a PC that acts as a server. Makes me want to dust off my coding skills to investigate further improvements to what is otherwise already a small but good and very handy program. With the latest 2.0.1 version I still can't easily scan for available MAC addresses. It seems to be pot luck whether it works or not. When it works though it's great and saves me lots of hassle.