In the relationship graph, did you select the checkbox for "allow creation"?
Let's take a step backwards. I don't think you're having Tab issues - I think your issues are more basic than that.
In FileMaker, every layout "sits" on a record in a specific table. If I'm understanding you right, it sounds like you set up one layout with a tab for each table and want "new record" to add a record to whichever table's tab you're sitting on. However, in that layout "new record" will always add a record to the table related to the layout.
There are various solutions around this. You could create "fake" tabs by having similar layouts that appear to be different selected tabs and have each sit on its appropriate table. Or, you can use portals and scripts to add new records to related tables.
But, Records > New Record will always give you a record in the table related to the current layout.
Am I barking up the right tree, or did I misunderstand you?
Very many thanks for your prompt replies. Fitch u hit the nail on the head. I was obviously too involved at a deep technical level which blinded me to the obvious! :0)
Very many thanks again