Cleaning out the cabin air filter, I ended up stripping out the area just below the windscreen where the engine management electronics are, and I noticed that the leaves and stuff seemed to disappear down the inner guard at the rear of the wheel arch, so I ended up undoing the bottom two fixings (hex-headed 8mm screw) and pulling out the crap. Jesesu there was half a bucket-load of soil and leaves in there - around the air filter there were olads of very dry leaves, which could have built up and become a fire danger. Beware. I have pictures, but can't work out how to post them!