Use the name box in the Inspector's position tab to give the Subs tab an object name.
Then Go to Object ["yourObjectnameHere"]
will select the desired tab.
(Hint: this can also be a useful way to keep a specific tab selected when a script needs to freeze the window, change layouts to process the data and then change back.)
Is it possible to apply this to a button from another table and layout? I have button on one table layout that opens up another table and layout via 'Go to Related Record' which is fine, but I would like the related record to show a non-default tab on opening.
Yes. Phil's solutions will work fine. However, instead of just setting up the button with GTRR, you will have to use a script that uses the script step Go to Related Records, new window (if you want to see it in another window). Then the step Phil mentioned with Go to Object, etc.
Great, thanks for your help. I'll give it a whirl.