I'm not an android or apple user so I had to buy a tablet specificallly for my Mavics and Spark, After soem testing of a few friends tablest I got a Lenovo Tab3 7. Has been working great.
UNTIL today.
I was prompted for an update and did so and now it just hangs on the start screen with the white Spark and blue flowers after a while bombs out with an erroe about non repsonsive or something and if I want to report it.
So short story was suposed to do some work filming today and couldnt!
How can I roll back to the previous version?

One thing you can try is start your spark after it has completed the start up sequence, hold the battery button till you hear three beeps.  try to reconnect wifi, then the app.  If the problem persists connect your device to the DJI Assistant app and you should be able to roll back to previous version.  There is a youtube video that shows you how .. just google it.

Yeah its not that it doesn't connect! It doesn't WORK! It just hangs in the start splash screen. Tried re-installing 3 times but it doesn't work. Its the olnly app installed on the device and the device has been updated.
I've gone back to 4.1.8 from APKMIRROR.com but not sure if it will give me grief soon about not being up to date.

Just verify, you upgraded the APP, right? Could you please take a screenshot of the start splash screen when the drone is connected to the tablet via WiFi?