Finally(?) found the correct solution to this issue as it had happened again.
I wanted to make an ISO file yesterday so I installed PowerISO to do it with.
Later on my step-daughter tried to play a CD but it wouldn't "start". I told her it wouldn't play because my computer has musical integrity (not quite true!!) but I had my doubts!!
So this a.m I went for a troll through the system and registry to see what I could find.
First off, all the Autoplay settings through the Control Panel were checked, all the options were still set at the default "choose a default" setting, the "Use Auto-play for all media and devices" was still checked so there was no changes there then.
Next stop the registry. I'd recovered one of my old WinXP "Elmers Tweak-lets®" notes that helped me with this one, so I wasn't running "totally blind".
First I went to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\cdrom and made sure the Autorun REG_DWORD value was set at 0x00000001 (1), which it was.
I then went to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer and checked that the NoDriveTypeAutoRun REG_DWORD was at the default 0x00000091 (145), which, IT WASN'T!! It was in fact set to 0x000000FF (255), which is the disable ALL Autoruns setting. So I changed it back to the default setting above (note that if you have a floppy drive the default is 0x00000095 (149).
After this I just double checked that the other two (on x64) entries at HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Policies\Explorer and HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Policies\Explorer were at the correct settings which, after changing the HKCU key entry they were.
After this I did a search for any other entries and I found one under a VMWare saved data key which had the NoDriveTypeAutoRun setting set at disabled 0x000000FF (255).
This would explain why my auto runs stopped originally. I'd had VMWare installed for a very short time last week but this key entry tells me that when it was installed it had taken over the control on how my removable media worked.
Now I can't say whether PowerISO did the same thing when I installed it yesterday as I had uninstalled it before I looked in my registry today but I have read (elsewhere) that it might/may/will do.
But now I've changed the keys back my auto-plays are running as they should do / I want them to again. In fact I've exported the default settings as a .reg file just in case it happens again.