Apparently Microsoft don't like my backup method.
I got a 0x800F0A12 error running the SP1 install file because "A disk management tool from another software manufacturer was used to copy (or clone) the disk or partition on which you’re trying to install SP1
Yes, I regularly clone the HDD because it has several OS on it and cloning the entire disk is relatively painless.
Microsoft's answer to solving this problem is to reinstall Windows 7. Not likely!!
Of all the OS on the HDD only one has a habit of wrecking the others when not being installed first - Windows.
I am certainly not reinstalling and I am sure many others when having problems updating will not update. So, that's another generation of Microsoft OS out there not patched and vulnerable.
I suspect the real issue here is that I am running one of the clone drives and not the original install drive. I will try installing SP1 again when I next make backups (yes MS, cloning is a backup method). There are only two backup drives so if after the next backup SP1 still fails it will mean the original drive is the other one!