[Chris152]

Found it, I think! It's right up behind the glove box, just a few inches beneath the dash, a green box. I tried to press it together as best as I could, very hard to tell as it's so far out of the way and I have to lay on my back looking up from the bottom of the footwell - I can't figure how to get the thing off its harness, and don't want to break it trying.
Anyway, it still won't start! Is the next thing to check if power is reaching the pumps? Multimeter arriving later today.

Pic in case it helps anyone else looking for the same - that's looking directly up from the floor pan.