Lost Focus Mk3 "Intelligent Access Key" (key fob) advice needed :-(

So, bought a shiny (nearly) new Focus last weekend, and the first thing my other half has done upon taking it for a spin is to loose the key fob :-S 

My first instinct was to see if there's a way of reprogramming/resetting the keys so that the lost key fob is "forgotten" by the car, making it harder (although not impossible) for someone to drive off with it, in the unlikely situation that they're able to find out where we live.

I used to have a Mk2 and this was a pretty trivial exercise to do with a bit of hokey-cokey moving the key back and forth in the ignition to put the car into program mode, but no such luck with my Mk3. The "Intelligent Access Key" is one of the 3-button fobs with a physical key hidden inside, and the car itself has a start/stop button, but no key hole - not even a hidden one, which I understand some models have.

After speaking to Ford, they were unable to give me any advice, and said I would have to wait until Monday when their service centre reopens, leaving the car at risk over the weekend.

So i guess my question is, has anyone been through this process successfully themselves? There are plenty of guides online, which i've spent most of the afternoon reading, but none seem to work. The 2 main methods advised are to repeatedly press the start/stop button, or a combination of pressing the lock doors button and tapping the brake, but neither have worked for me :-(

The car is a 2018 Focus St Line 140, with start/stop engine button, and intelligent access key. 

Any advice would be a massive help - thanks!