The depth adjustment wheel is free!
So I went hunting for butane blow torch, but no, nothing doing in these parts in the evening. So, having picked up a consolation prize of a set of screw drivers (I realized the value of the right size screwdriver while doing this plane restoration) I went back to my shop pondered the problem.
My wife had mentioned we had a gas fire lighter somewhere so I went hunting for that. While I was rummaging around in the dining room drawers I spied a big bag full of 8 hour candles. Perfect! So I set about heat treating the adjustment, then having at it with a pair of pliers and the wheel wrapped in cardboard. But no, after several attempts, and numerous liquid wrench reprisals, nothing was moving.
Then I remembered we had one of those jar wrench affairs, the ones that have a rubber strap that wraps around a jar lid and threads through the handle of the tool which allows you to prize off tight jar lids easily. Hmm, nice try, but that didn't work either.
I decided to pack up for the evening and gave the wheel a last liquid wrench soaking. Then I spied the small c-clamp that I used to afix a small board to my pegboard to make a makeshift shelf. I clamped the adjustment wheel fairly firmly, but not enough to actually damage the wheel (sadly a couple of plier slips in previous attempts has already marred it), then I just grasped the clamp like a handle and twist! Easy as pie.
So now of course, having unseized the unseizable I find the very last screw that was behind the wheel is also seized. Frankly I don't even know what this screw is or what it is supposed to do. Perhaps I am missing a piece of the plane.