I charge my batteries overnight but when they are not being charged it would appear that
They are not available for use until the charge time has been met. For this period I am on mains
Power not battery. Is this correct or is there a software fix required.
This is how it works. Without setting up automation with other products / services then you could shorten the charge period to an average time when the batteries are full.
Trumper I think it's a reasonable assumption that if you are charging from grid, it's an off-peak period of your tariff, and therefore even once the batteries are full, it makes sense to run the house from grid rather than battery.
There is a case where this breaks down... if you are charging prior to a peak-rate period in your tariff (eg 4pm-7pm on agile or flux), the grid is not particularly cheap, but cheaper than it's going to be. Which firmware are you running ? I did discover an odd feature that might be useful for this scenario, but it is probably very firmware-specific. (I'm on 309 on 3rd-gen hybrid.)
See https://community.givenergy.cloud/d/5202-battery-charging-when-set-to-export-at-percentage