I spoke too soon Cubasis refused to open again this morning!
Only real similarity is that there were about 8 audio tracks with two tracks of cut up drums and a reasonable number of effects. The undo function took a long time to operate but the operating system was reasonably responsive. I closed Cubasis when I finished editing the track I was working on. When I tried to open it later it wouldn’t open as described above.
Once again a re-install was required to fix. This time however I backed up and saved my data 
I checked for a crash log but even after syncing my iPad the folder was empty and there was nothing for the previous failure? The project also opened fine from the back up so it was not corrupted.
Once is unfortunate, twice makes you begin to wonder if its worth investing time in the App at this stage…?