I finally went to the podiatrist last week, and - I'm sure you were all on tenterhooks - she didn't mention weight at all, even though I was wearing shorts. Which Mum said she wouldn't. Mums are wise.
The short version: I have the feet of a baby. And I do need orthotics. I'll get them in a couple of weeks.
It seems I'm significantly hypermobile (though still not so bendy as the siblings). That means that my ligaments are stretchy rather than taut, so my feet don't support themselves, which is why the arches are flat. Also, my knees bend backwards and have a tendency to "lock" when I stand. I've always known this, but apparently it is very bad and I mustn't do it.
When babies are born, their feet are flat and tilt outwards a bit. (Imagine a baby's feet dangling as you pick it up.) As children grow, their heels and the balls of their feet tilt inward and downward, leaving the arch of the foot higher by comparison. But my feet didn't make it all the way, so that effectively leaves me with feet that are still tilted, and which roll in when I stand, causing me to overpronate. And that's what's causing the pain.
As a secondary result, my big toes aren't working properly because they're at the wrong angle.
The podiatrist was very thorough. She examined my wrists (to see how bendy they were), the pattern of callouses on my feet, and the way I stand; she got me to walk up and down for several minutes; she drew lines up the backs of my heels and calves and examined the angle at which my foot rests. Somewhat worryingly, for my foot to be at a natural angle, I'd have to tilt it outwards far enough that none of the toes rest on the floor. The orthotic inserts will fill in that gap so each foot is level.
They'll be made of polypropylene (which sounds a little sweaty to me, but there we go) and I'll get them at the beginning of June.
I also have exercises to do: calf stretches (familiar to the runners among us), balancing on tiptoe, standing on one leg (well, one at a time) with my eyes closed, and walking around on my heels. This last makes me feel, and no doubt look, very silly.
But it's doing something! My feet don't hurt when I get out of bed in the morning any more.