This could be the same issue as the rusted plenum, however it sounds like the heater core, mine has just gone.
Try running water in the plenum grille below the windscreen only. If no water, clamp the heater hoses under the bonnet to see if it is still happening.
You should smell coolant if it is the heater, and you said it smells a fair bit so sounds like coolant, also your coolant level will be dropping all the time as most of the coolant will be going out of the drain-hole that comes out of the firewall, (under the bonnet), that comes out just above your passenger side chassis rail, with a smaller percentage going into your footwell.
I re-plumbed the inlet and outlet back into itself under the bonnet as a temporary fix, and will try the Resolene system sealant when I get a chance.
Your heater won't work but it stops it leaking as a quick fix.
Tim9 got a reaction from bear351c in Floor wet after wash, Dripping near brake pedal and handbrake bracket