That's the very update needed. When it fails to install you will see an error :0xabcdefgh where 0xabc... is the error code number you need to google for the solution
Thanks. From a preliminary glance it looks like this is caused by buying a PC with Win 8 pre-installed and updating to Win 8.1 a few days later either from WU or the app store and it requires some sort of Refresh to fix it.
I was waiting for later this year to buy a new Desktop with Win 10 pre-installed. After this episode I wonder whether I should wait till they come with Win
10.1 pre-installed rather than upgrading to it later

.
Funnily enough, this is the 1st time I've switched this PC on since your reply and now the little Windows icon in the taskbar has appeared, altho' KB3000850 is still failing and I'm still not running Windows 8.1 UPDATE. I guess this is because MS has officially announced Windows 10 now available as free upgrade.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC