OK so I have managed to get this to work !!!
Now I cannot truly explain why which disturbs me greatly.
This is what I did....
I firstly copied the library source into the project rather than using the external library.
This actually made no difference and it still failed at the install point.
Then I changed "tmp.apk" to "updated.apk" and all of a sudden it loads :O
Interestingly Erel's example on loading an APK from the assets directory did work OK on both my devices, and the reason I changed the filename was to use the download part which worked well and then try to copy it locally then update.
But I never got any further than changing the filename and recompiling to see if the file was correctly located in the file system.
I will do some more testing and check that this is a consistent 'fix' on a new device, but I don't feel this is really the issue which as I say really worries me.
Anyway thanks for taking time to help its much appreciated and I hope this works from now on
Harry