Update - December 28
Having a couple of hours free this afternoon, I've been to take a look and start the repair work. On arriving at the door, I found there was a very large gap underneath it that could easily be squeezed through (see pic 1), the hazard tape was broken, and the door wasn't bolted - to whoever has been going in this way, all I'll say is which bit of "do not use this entrance" don't you understand, and would you like it explaining in words of one syllable whilst being hit round the head with a shovel?
First job needed was to clear all the old soil and leaf mold from around the door; this is a job that really needs doing every couple of years to keep a free swing. Ten minutes with the folding shovel had this sorted, though I upset a lot of worms in the process. Next, I had a good look at the inside; the large boulder and its surrounding rocks didn't look to have moved, it was the choss on top of it that had gone downslope and caused the hole.
There was plenty of rock in the entrance chamber so I set to work creating a packwall to fill up the gap and spent the next hour walling. As can be seen from photo 2, I've got the gap mostly filled up but not completely, so more work is needed and mortaring up to really secure everything. In the meantime, please continue to only use the upper entrance. I also think there's some work needed on the big stones below to make certain they don't move. Photo 3 shows just how big the hole under the door had got!