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Just to let you know, Buenchico and Sycamore, that everything is now as it should be with the printer / scanner - without even having to resort to contacting HP!
I repeated, for the second or third time, the uninstall / reinstall procedure and when this didn't make any apparent difference, I thought I'd try the ePrint facility. I duly opened this and somehow or other it worked! I've no idea why but I'll just accept that it does! So I can now scan either direct from the button on the printer (both in wireless and USB mode) or from the Devices and Printers screen on the PC. As already mentioned, the iPad app also enables remote printing.
The only sting in the tale is that the Start ¦ All Programs ¦ HP ¦ HP Envy 4520 series still shows "(empty)" rather than an options submenu. At the moment I don't need this so I'll stick with what I've got!
Thanks again to you both for your help, ideas and patience over the weekend.
JG