My ISP doesn't throttle connections, so they say, and I've no evidence that they ever have. Anyway, I've tried via my own UK ISP and also via FON, a Spanish network to which I have access, represented in the UK by BT.
The result has been failure via each! :-(
It would seem to be either a problem on my iMac (yet Disk Utility, TechTool Pro, Memory Clean 2 etc, report nothing amiss) or with the particular Apple target server to which my Software Update points.
Incidentally, I have tried direct via the Software Update panel in System Preferences and also going via the Apple UK website with the same result.
I shall let the dust settle for a few days and then try again.
