Seem to recall most of my old cars went through a stage of the heater matrix leaking into the passenger footwell, but not for a few years/cars now.

Inside of the windscreen frozen up this morning, so clearly very damp in the car – lifted the mats and the passenger footwell is damp, so assuming that is what it is?

That could be blocked drains as well. Google it for the make/model and you’ll see if there is a common cause.

They can, <2008 Berlingo’s did it.

Although if it’s damp enough to notice, you would also notice the coolant level dropping in the engine.

Plenty of other causes, the drains in the scuttle/windscreen being blocked would be #1.

My Berlingo was soaking wet on the drivers side (it would accumulate at least a gallon before the water level got above the carpet). I think because there was a rubber lip seal around the top of the door, it wasn’t the actual door seal but I think it probably kept the wind from blowing rain in against it under pressure and forcing it past. Never managed to get it to seal properly so if it hadn’t been scrapped I’d probably have just drilled a drain hole under the carpet 😂 It wasn’t the heater and wasn’t the scuttle drains so I gave up and just cut the carpet / foam out of the footwell so I could remove it and dry on warm days, it was a shitbox so i didn’t care.

That could be blocked drains as well

Yes, sunroof drain pipes run through the A pillar. A blockage, usually at the outlet over the wheels tends to back up and wet out footwells.

whats the car

many cars suffer leaky door membranes , leaky high level brake lights , rear boot seals etc etc.

drain holes are more likely, if its the heater matrix you’ll be down on water pretty fast. Some cars block up under the windscreen air inlet, others under the sunroof/boot/lights

Its a 2015 Fabia – will investigate drain holes in daylight

Does it smell of coolant? If it does then yeah, could be the matrix. If not, as other folk have suggested, drain holes. Normally the scuttle panels get full of leaves and water ingress in through the pollen filter housing.