Peugeot 607 Fuel Flap Fault

Sounds familiar. I had something similar happen on my 607. Check the electric actuator. You'll probably find that's the culprit. Does the central locking work normally?

When you say 'electric actuator', are we talking about the lock actuator itself, or something else? And yes, central locking is working perfectly fine, which is what makes this all the more confusing. Whereabouts is the actuator located? Is it easy to get to without dismantling half the car?

Yep, the lock actuator itself, the one that physically moves the pin to lock/unlock the flap. Since your central locking is fine, that points even more strongly to a problem isolated to that specific actuator. Frankly, getting to it is a pain. If you're not comfortable poking around with trim and wiring, I’d suggest a workshop. They’ll diagnose it properly and replace the faulty bit.

I was hoping to avoid the workshop but it sounds like the best course of action. Turns out you were spot on: lock actuator. Workshop confirmed the diagnosis and replaced it. Cost me 210€ all in. Thanks for setting me on the right path!