I think I have made some progress - it was not using the persona I intended, so was defaulting to another persona that had “bot” enabled. I have turned off “bot” for the other personas now, so it is using the persona I wanted.
There also seemed to be a problem with having a user assigned to the persona I was using. When I removed this user, I started to get better results - and was getting different behaviour depending on which “Enabled Tools” I had set for the persona..
Will carry on investigating…