Great news! I think I've fixed it!!
Given the thread you linked Maurice, I decided to revert to assuming that there is a blockage rather than a split or tear somewhere. I poured water all over the offending corner of the roof (with it closed) and no miracles, it was still pooling inside behind the seatbelt assembly, beneath the rubber box I have been refering to.
I then sucked all this water out with a vax and connected a length of hose pipe to a funnel so that I could pour water directly down the drain hole in the drip tray. It still leaked in the same ppipelace so I was confident that the top drip tray is not the problem.
I tried to poke a proper pipe unblocker (steel band with a spring head and neck) down from the top and it was still reaching a complete impass. I decided to take another look from the other end so I jacked the car up and removed the rear wheel and plastic arch lining. I then attempted to push the unblocker through from the bottom and again it reached the impass. The pipe in question is made of white translucent plastic so I could see that it was making its way round the bend in the pipe, but not quite reaching the cavity beneath the drip tray.
After some pushing and shoving from both ends, I heard a rattle and on pushing my little finger through the drain hole in the top I could feel some sort of plastic clip rattling around in the cavity beneath the drip tray. I pushed some electrical flex up from the bottom until I could feel it appear in the cavity so that the clip would not fall back down the tube. I then spent ages trying to fish the clip out which I finally managed to do with a piece of blu-tack on a pair of tweezers which were attached to a piece of string. (Taking no chances here!!)
I have attached a photo of the offending clip and was hoping that someone would be able to identify it for me. I'm guessing it might be the same thing that Pablo found. It would be good to know whether it's just something susceptible to falling down the hole in the top drip tray or actually part of the assembly that sits beneath it.
I thought I could upload the image but I see it's a URL so I'll add this later....
Anyway, as you can imagine I was absolutely extatic and immediately repeated the experiment with the funnel and hose. This time, water was lashing out of the bottom of the car (much improved flow rate) but to my horror, water was still leaking inside the car in exactly the same place.
I resigned myself to the fact that something must have come adrift beneath the drip tray, sucked the water out again and put the arch lining and wheel back on. I then stuck the roof up and repeated the experiment where I just poured water all over the back corner of the roof.
This time no water whatsoever made it inside the car! I repeated with lashings of water and again, nothing made it inside the car. This leads me to beleive that there is a limit to the amount of water that the system can drain. I guess this must be based on the amount that can realistically get in via the gutter on the back of the roof when it is in the raised position. I'm assuming that there is another open sided drip tray beneath the top one which backs up if the water is poured in quicker than it will actually happen in normal use. CAn anybody confirm this for me please? I don't really understand why it wouldn't be a closed system once it gets beneath the drip tray but in my experience, it obviously isn't!
Anyway, my problem seems to be solved so I'm back to the garage tomorrow to see if they can justify the £60 + vat that they charged me for "unblocking" the drains!
To be fair to them however, they did give me some good advice on how to dismantle the centre console in order to remove the carpets. They even offered a telephone consultation if I got stuck with this.
I'll obviously ask if they recognise the offending clip and see if they can confirm the practical limitation to the drain system.
Thanks for your help with this Maurice. I will probably cross ref this thread with Pablo's just in case he still checks it and can confirm my suspision about the clip.
Regards,
kjs