So at long last I have icons and a start bar showing on my desktop. I am assuming one, many (or none!) of the following actions I took fixed the problem. I'll leave it to others more experienced to speculate which
Re-imaged existing SD card, using a different mirror source. Ensured 'safely remove hardware' option within windows used before 'unplugging' card.
Selected 'Expand root partition to fill SD card' in Raspi-Config splash screen and then rebooted. (did not realise previously that option had to be selected rather than just showing on the display before exiting).
Ran sudo apt-get update and sudo apt-get upgrade. (this had previously failed with a reading package update error - but worked this time). Rebooted.
Logged in, ran 'Startx' and hey presto! Full graphical desktop present and working.