The earlier demo showed two project gantts - one working, one not working (because max rows set to 200). The latter was under the resource gantt tab simply to differentiate the copies. Sorry I wasn't clearer!
The good news is that it is fixed - largely thanks to your help making me look at this differently. The problem was that row 36 had the parent_id set to activity_id rather than set to null. So Max Rows set to 15 (or presumably to 35) worked fine but fell over at 36. In short the problem was in the DB.
Thanks very much for all your help - success!
Good for YOU! Hard to see some of these. Another chap set task_id and parent_id the same so it was finding all these circular resource groups and could never display anything because there was no topmost parent task.