Water leak on a Fiat Croma

The nearside footwell of my Fiat Croma has severe water ingress - what could be causing it?

Cromalogical order

I have had many trouble-free years driving Fiats, my current car being a Croma 2.2 auto (a superb car in most respects). However, it has recently developed an annoying "intermittent fault" in that the...