Yes, you saw right, the title of this post. This section which has given me soooo much trouble, at last I feel good about it.
I was right about where it was wrong, I had added a left hand to a piano solo which really didn't work. In a stroke of inspiration I moved the left hand stuff to the mellotron, and hey presto! After that, lots of little tweaks, humanising programmed bits, balancing and so on, finding a few other bits which were weak and strengthening them. Finally I actually think it's not only acceptable, but something to get mildly excited about. So now I'm listening to it in the context of the previous sections.
Then came the problem of mastering it off onto an MP3 file. After 4 attempts of it crashing I realised there was something fundamentally awry with the cubase project, and so I went through the full process of creating a duplicate. I could copy the MIDI note information using cut and paste, but all the volume tweaks and settings, and instrument tweaks and so on had to be replicated. That took a while but finally I have succeeded.