|I've been unable to get the information you ask for - nothing coming up on google search which actually gives the size and type of battery.|
To ensure you get the right type and size of CMOS battery, remove the laptop's entire bottom casing and remove the existing one. Then take it to a store which sells that kind of stuff.
You're gonna have to open your laptop casing anyway to put the new CMOS battery in, so you may as well do it now to get the old one out.
If the existing battery is already flat, it won't make any difference how long you leave the laptop without a battery before you manage to get a new one for it.