nested tables - padding?

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I have a structure using nested tables (don't panic, it's for tabular data)

http://fitzgraham.netne.net/wordpress/timetable/

Trouble is, I need that border to continue all the way and beyond the base of the bottom table.

at the moment, the spacing (which is required) is achieved with margin, but when I switch to padding, the gap disappears (using Firefox, untested elsewhere)

I would be loathed to using blank lines, as it seems a little hacky.

any tips?

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Above is what I would like it to look like when finished.