Hi,getting really serious amounts of water on the driver side of my corsa?

Hi im getting very serious amounts of water comming through the insulation and carpet on the driver side of my 01 corsa, i took it to a friends garage and tought we had the problem traced back to under the bonnet we sealed the area but the problem continued.

My local opel(vx) dealership are a complete crowd of robbers who wanted to charge me 291.72 to put in a o2 sensor so i hope you can understand why i dont want them to sort this problem. if any one knows anything that could be causing this and any help on how to fix it please reply:o thanks