Leak Into Passenger Footwell!!

[Bumblebee Puma]

Hi, I have had this problem for a while now where everytime it rains the front passenger footwell fills up with water! I have now pulled the carpet back and have noticed a slow leak running down the left hand side (past an electrical unit which is very worrying!) I know nothing about cars myself but would like to sort the problem out without having to take it into a garage if possible! I have changed the pollen filter as someone said it might be that but it still leaks. When I poured water down the windscreen it does seep into the pollen filter. Should this happen? I have silicone sealant that I can go around the edge with. Is this worth doing? I am thinking that maybe some drains might be blocked with sludge but have no idea where to look for the drains! Please could anyone help as it is starting to rain an awful lot and the car isn't smelling too nice! Many thanks