So today, I shimmed and clamped the hatch in place and commenced screwing the hinge down. Snapped off one drill bit while drilling pilot holes. then snapped the head off a brand new stainless steel screw.
So instead of residing on the TD, my hatch is now resting on the table. I had to remove the hinge, chisel around the broken off screw and bit, get them out, and fill the craters created. The broken off drill bit was easy enough, I just chiseled out enough timber to get a pair of pliers on it. Extracting the 1.5 inch screw was a particular PITA.
I tried pliers, pincers, vise grips, side cutters - nothing could grip the visible 5mm of screw enough to turn it out.
Things were getting desperate. I'd pushed my arthritic hands as far as they could go, and I simply could not grip it hard enough.
Then I had an idea. I grabbed the battery drill, and managed to get the three jaw chuck on that tiny bit of protruding screw at the bottom of my little chiseled out crater.
Didn't work, tried again. Then 3rd time lucky the screw came out.
Mixed up some filler and filled the craters created by the extraction dramas, and ran away quickly.
I'm sure it'll all look easier in the morning.
For the record, it appears I have managed to run into staples in the timber, causing the breakages.