So this is on the SE trim. After starting and setting off, I get a click from somewhere near the foot-well as the car passes 40kph (about 25mph). This only happens once per driving session. It's not the doors locking, and nothing is obviously going wrong. Any ideas?

It's nothing to worry about. I've noticed it too and it comes from the brake pedal. And yeah this only happens once after you start the engine. It feels like something is engaging (ABS perhaps?).

It's normal as Malediktion said. I think the sound comes from fuse box (I've got a LHD Fabia and the sound comes from end of dashboard near the door). It's got something to do with something engaging, I'd place my bets on airbag system

Remember as well as auto door locking, the ABS system will be getting a self check once the car is on the move.

The doors lock?