My GT had a good cup full of water in the driver's footwell, just before Christmas. After reading here for possible cures I first removed the external trim panels at the base of the windscreen. Snapped off two of the push fit fastners in doing so, mind you. (Can anyone advise how to get these out without risking snapping them?).
There is another plastic trim tray under the main one and the drain is under that. About 30mm diameter with a plastic insert. This was completely clogged with organic matter, including fine roots!
Cleaned it out, replaced the trims, (after wiping the visible trim with groundnut oil as separately advised on this forum) and cludged it all back together (minus 2 fixings).
I sucked up as much moisture as possible from the footwell with a Vax and then left a fan heater running in the cabin for about 6 hours. Result - footwell dry and remains so. Thanks to all advisors on this forum.