How much work is required to replace a heater matrix?

I have a 2015 Ford B-MAX and water is coming into the driver's footwell. Our nearest Ford garage (which has been very good and I trust) has told me that it is something to do with the heater matrix - either...

My car only blows cold air from the heater - what's the problem?

I have a 2010 Vauxhall Astra that is blowing cold air only. I have checked the coolant levels and they're fine. What could be the problem?