I'm also in Kent. Must be the rain. I drive around with newspaper under the mat and it's usually soaked. My RX400 also has a leak, driver side. The sunroof drains are ok, as are the door drains. I've put newspaper up below the dash and there's no evidence of water dripping down. I'm sure it's coming in behind the carpets. There was no problem in the summer, even after rain. It's almost as if there's a container that fills up because it can't drain and overflows and in the summer it has time to dry up.
I put clngfilm over the wipers, at the back of the bonnet, to block water going in and it did a little. If I pour water down the drains at the side of the wiper well it comes out under the car, as expected.
My local independent who is very good, can't find anything and are not keen on digging too many parts out.
