There are ways to hide and reveal a field or other layout elements.
You can create an invisible tab control and give the tabs object names. Tab one is empty, tab two has your fields or other layout objects. Use a script (can be controlled with script triggers) and the go to object step to select tab two to reveal the hidden layout elements.
You can place a portal to a single portal row (resize the row to be much taller than the default if you need the extra space.) Now you can hide and reveal items in the portal by changing a value in the parent record's key field so that their either is or isn't a related record visible in the portal. (And in FileMaker 11, you can also manipulate a global variable so that a portal filter hides and reveals the related record.)
Thanks for your help (again!)
I take the point re: tab controls, but feel it is a slightly cluncky way of showing/hiding things. Maybe I'll post it in the feedback forums.