Thanks for the reply.
I hadn't understood properly the relationship of fields belonging to tabs.
So, in my case, when I had a field belonging to a tab who's content appears higher than that of others, it wouldn't matter how much I changed the 'order' of the antispam-captcha field, it would always be restricted to the bottom of that tab's elements.
Now I've changed the tab that the field belongs to and everything is appearing how I want.
Many thanks for you excellent help.
Dave