Reckon it’s appropriate to provide an update.
I bought the
Screwfix scraper thingy and attached it to the drain rods, taping all the connections to make sure I didn’t lose anything, and pushed it into the gap. The scraper is much smaller than the gap’s width so it took multiple “drags” to remove a decent amount of muck, but I made progress. Eventually I got bored and decided to check whether I could squeeze into the gap and to my astonishment, the answer was Yes, though it was flippin’ tight. So I took a length of 4x1, squeezed in and dragged out as much of the remaining gunk as possible, then blasted out what was left with a hosepipe. All this activity revealed a drainage channel on the floor of the gap, so from now on I should be able to keep it clear with the occasional jet of water.