Just done this, by removing the trim as in the video. Pretty easy - for the three pins, you get your nails under the inner head and pull, which releases the fastener. The sill trims just pull up, and the B post top trim needs lifting slightly to allow the rear panel to come out. The basket is easy to reach this way. Mine had a couple of small twigs in there after three years, no blockage at all, but it lives in a garage and very rarely stays out under trees.
I had a go at just reaching down to access the basket, but hadn't a hope in hell of doing it that way. With the door wide open and the seat slid far forward with the back tilted forward, I could get my head in to see the basket and work it all out.
Roger