Just changed the batteries in both my fobs (MY20)...
After changing the each battery the fobs didn't work

However, putting the fob in key-fob-battery-flat slot (low down on LHS of steering wheel) and pressing the start button to power on the car, then waiting ~5 seconds, the car and the fobs seem to have re-paired themselves - happy days.
It did take a couple of attempts, but I now have 2 working fobs again.