I pulled into my drive, about to switch off the engine and got "key battery needs replacing" message (March 2018 car now has 9500 miles on the clock).
got the other key from the kitchen and locked the car.
Next day bought a strip of CR2032 Panasonic batteries from Waitrose.
Opened the key ,removed the battery cover and removed the battery, inserted the new battery and found it wouldn't stay in the housing, flipped it over and it clicked into place.
Key didn't work later found out at the dealer that I had it upside down, (the side with the identifying text should be uppermost)
The reason the key battery wouldn't click in the right way round was that the tiny retaining stub had broken off. It was however possible to refit the cover and click it into place successfully.
Note: The original battery (Sony CR2032) and the new replacement both showed just over 3 volts on a multimeter!
Using the 2nd key with a new battery fitted last night, parked in the drive and selected park, immediately got the "key battery needs replacing" message again.
So now i realise that the only way is to take both keys with me to avoid being stranded.
You can use the flip out key to get into the car. by using the metal key to remove the plastic cover on the left segment of the door
handle to expose the manual key hole.