OK. Now I have tried running Create on my win10 desktop and it works fine with both boards. The one I had the problem with running create on my chromebook installed on on the desktop but was set to port 4 as it was on the IDE, There seems to be no facility for setting ports on the chromebook installation of Create so I guess this is a bug (or feature!) of the system.
It would be very helpful if there was a way around this.