Thanks very much for your very helpful (and complete) answer.
I had already decided to try doing this using the raw api so I could get more debugging data so I used yours and cut and pasted the app details straight from the web browser. Surprise it worked.
Long story short: I had mistakenly though that the client_id was tied to the project owner and that it was the same for the two projects. ( there was one digit different and I am highly non visual so I did not notice). It took a another hours stuffing around before I finally worked out it must be the client_ids.
Usual story: problem between chair and the keyboard : )
Apologies for the noise!