Leak inside Vauxhall Vectra

Okay, on Sunday morning when I got in the car I found that I had about half
an inch of water in the front and rear passenger side footwells. Thinking it
was the heavy rain/wind we had the previous night and that I may have left
one of the windows slightly ajar I just mopped it up and thought nothing of
it.
Tonight when I got in the car I noticed it is now an inch deep (I go in
early so it was dark first thing this morning).
I cannot see any obvious leaks but also cannot understand how much water can
get in so quickly.
Can anybody give me some obvious places to check where water could be
getting in please?

Many thanks in advance

Tony