Thanks Jock1e for your advice. In fact I had AVG and Bit Defender sequentially and uninstalled them with Revo uninstaller which is quite thorough. I did though follow your advice and got uninstallers from their websites and ran them with what appeared to be little effect. Going to services turned it on for about 1 second and then off as usual. What seems to be going on is that once you have used an external antivirus (Macafee was on the Windows 8 DVD) it won't go back to Defender.
Any thoughts on a registry tweak?