Thanks Tomcat. CT18 was indeed quite caustic a few years back but apparently was updated to be pH neutral and non-caustic. It reportly used to result in dulling of metal but these days doesn't seem to have any such effect. One of our bigger accessory dealers in Sydney recommended it - saying "you can spray it all over the bike." However, I only use it for the lower engine, wheels and lower fork areas, just in case. Then I use a milder wash for the rest of the bike. So far, so good.
Regardless, I'll look out for Purple polish and give it a try. Thanks for the tip.