Yep .... well almost. I didn't forget ........ I CAN'T boot from the Install DVD, as I explain in my original post.
From what I have read, people seem to be able to install Old over New by simply loading the Old install disk in New and Old will be dropped over New. You cannot do this with SL when you are using ML because the SL installer will not run in ML. (I got it slightly wrong). Therefore you have to try and boot from the Snow Leopard install disk, which I cannot do.
I suspect that this is because I am using an iMac which has a Bluetooth keyboard and Mouse. By the time the Bluetooth wireless software is activated and transmits the "CMD + R" keys, or the "C" key, or the "Alt" key (On UK Keyboards it's Alt, not Option), or whatever, it is too late. The startup process has passed the point at which the manual startup key depressions are processed.
Having investigated this further, it seems that the Bluetooth keyboard and mouse can cause a number of problems around startup and wakeup (after going to sleep). Back In The Day, I had similar problems with infra-red keyboards and mice.
I have a USB keyoard, but it's a Windows layout and does not seem to respond at startup in ways that OS X understands. I guess I'll have to buy a USB Apple keyboard and give the Bluetooth one a Viking Funeral. Maybe, while I am at it I will scrap OSX (it's going to become a glorified iPhone) and install Ubuntu.