After reading the last post and following a recent upgrade to my Windows 10 system I decided to give this one final try.
Prior to downloading the app, I visited my user setting and deleted the Cisco, Sky and Sky Go folders. After downloading and opening the app the green play button appeared together with a message that an update was available, just like it did so many months ago when this problem first materialised. Anyway I clicked update, re-opened the app and waited. To my astonishment I am now watching Sky Sports! No black screen, no making the box smaller, just the same screen that I fondly remember from so many months ago.
Thank you Sky for months of wasted effort.