Front passenger footwell.
Remove the top side piece of horizontal plastic that runs under and alongside the gear lever to where the passengers feet would be. Kind of 'pops' out at 2 places.
Once you've done that you will be looking for a long, thin black plastic flap about 14" long which lies at the back of the heating control panel.
You kinda have to stand on your head to find it the first time.
Press the bottom tab down, pull the flap towards u and then disengage the top part.
Pull the edge of the filter - looks like a rippled hoover bag - all the way out - you might need to press the top part of the filter down as u pull it out.
Once you've got it out you might find a way, with one of the vacuum cleaner attachments to get in there and suck all the bits out - I've got a Miele so there was no problem whatsoever - in fact if you've got that brand u might need to turn the suction down to prevent possible damage.
A mate of mine has added a thin piece of transparent hose pipe onto the end of the nozzle and drilled holes into the pipe - that way he has a small, flexible way of getting into these kind of very restricted areas - works really well.
Good luck.