Went to the app store, appeared to update but happened far too quickly; asked me to reboot, I did so. Suspicious, I went back to app store and... the update link was still there. Tried again, selected reboot and got the message 'Available updates have changed'. Fair enough. I closed the app store fully, reopened. Tried updating again. Rebooted again. Didn't work, again; the update option still there in the app store. Gave up and downloaded manually. Will report back shortly if that doesn't work.
https://support.apple.com/kb/DL1845?viewlocale=en_US&locale=en_US
I tried to install manually but it gives me an 'Instillation failed' message. Hmm. I will try unplugging some of my other hard drives, in case it's an EFI partition confusion.
Unplugging my external drive appeared to work - appeared to - the manual installer appeared to do its thing, very quickly. I rebooted as asked and... the app store still has the 10.11.1 update tab. System Software Overview reports:-
System Version: OS X 10.11 (15A284)
Will try update Clover and changing model number in config.plist, which has worked for me in the past to solve different issues, while sometimes creating new ones.
Okay, updated to latest build of Clover and changed my computer's model number in the smbios section of the config.plist from iMac11,2 to MacPro6,2 .Not ideal, as in not a match to my hardware but, as with other issues, it worked, via the app store (this time on reboot the screen went black and an installing bar appeared, updated in a few minutes - and I will no doubt see if changing to MacPro6,2 introduces instabilities elsewhere. Fun and games. In System Software Overview it now reports:-
System Version: OS X 10.11.1 (15B42)