Right...
I removed the rubber seal and had a go!
I managed to get my arm in with a pair of pliers and tried to grab hold of the grommit. Unfortunately the same thing happened, and it fell down the back!
God knows how to find it again?!
Anyway - I pushed my finger in and tried to find the 'slit'. I didnt seem to have one, so I pushed a pocket knife in through the foam and then pushed my finger in to expand it a bit and make sure it was right through. I could then feel metal when I put my finger right in.
I threaded my cable in downwards and pushed about 1.5 meters through the hole. I undid the 2 thumb screws in the footwell which allow the foam top bit to be moved. I then pushed my hands up and grabbed about - and felt nothing!
I unreeled my cable and tried again. I must've tried about 15 times in total, but every time there was nothing there. Eventually it started raining - so I quickly put my rubber seal back in and gave up! My cable is about 4mm thick and pretty sturdy.
My arms and hands are all shredded and bleeding

(the inside of the metal bits are sharp!) and I'm no further forward! I'll need to try again soon - and see about filling the hole from the grommit with something!