Microsoft Support ran through many of the steps in Apps and Tiles Not Working or Responding: Fix in Windows 8 but I had already tried those. They then suggested creating a new user account but that solution had only lasted a week for me.

MS then downloaded the Windows 8.1 installation files into folder C:\ESD and ran an in-place repair install from those files. On completion which took ages, Windows 8.1 reported as unactivated. MS then provided a new product key and all is fine. The apps all work (at least for now) and all my programs and files are intact. I had to update my screen personalisation and the locations of some Outlook default folders but that was all.

The product key differs from my Dell OEM key. I don't know if it is a retail key or something else.

I've also created a bootable Windows 8.1 USB flash drive by the following procedure. I used the program ESD-TO-ISO to create an ISO image. See Option 3 in Windows 8 and Windows 8.1 ISO - Download or Create. Note I had to copy the program and directory to the root of my C: drive before it would run. I then used Rufus to create a UEFI bootable flash drive from the ISO image - see Option 1 in UEFI Bootable USB Flash Drive - Create in Windows