Thanks MartyTX & JSWolf.

I muddled through and got my books form Calibre library to Marvin on the ipad via iTunes. Not sure which actions did the trick - much switching off systems and restarting was involved. Now to try for a more direct Calibre to Marvin method.
A general comment: I have been involved with computer systems for over 45 years (work at a university) and have designed though not programmed educational utilities. I have learned to be persistent and to experiment with purchased software, almost every bit of which involves these sorts of complexities and trials. It's like ordering a new car and it turns up in boxes for you to put it together, when all you wanted to do was to drive it. I believe the vast majority of people are not computerniks: they want to do things like collect and read ebooks, and would give up if you even mentioned a MobileRead forum.