When I turned on my F3 today I got this message on the LCD screen: "Backup battery end please change". Surely I could not have used it up in four months? I had my EX1 for two years without having to change this battery.

What should I do?

What battery is it? BPU30? or 60?
The 30's weren't recommended.

Ola is talking about the backup Battery not the Main battery... The backup is probably just a CR3032 3v....That is strange that you got that message but fingers crossed it is just a bad cell out of the gate... just swap it out and it should last for years just like in the EX1. Hopefully we do not have some strange battery drain issue again except this time with the backup battery instead of the main.... :) Keep us updated if you swap it and are warning free or if the issue eventually returns.