This is what it looks like from the top, the hood had been removed from the car. 
The second picture was taken through the rear window [zipped open!!] and is looking at the passenger side.
Whatever you do, don’t push too hard, or you’ll disconnect the tube from the tray. If this happens there’s a lot of work involved in dismantling the area behind the seat to get at it.
Try finding the bottom of the drain tube and pushing something up from the bottom,
I find a piece of old flexible net curtain rod is the best, or if you have a Mk 1 with the original aerial, it seems made for the job.
There is a sort of ‘flappy thing’ at the bottom of the tube, it’s designed to keep noise from getting up the tube and into the car, but it can allow bits of debris to get stuck. 
Drain holes in the sills 
Here’s what the drains look like when they are completely removed from the car
