Don't think they ever put out any updates for firmware.
The one they provided on their old page was 11.831.03.00.00.B427
This is the old page - http://www.cellc.co.za/internet/drivers
Go to the "downloads" link on the left
The thing with v23 is its ok to check balance with 8ta as you can do via sms but atm V23 ain't got USSD enabled so not too helpful for checking CellC balance.
Working on enabling it but theres some issues going on.
Ginggs re V23 - you now can't add files direct to the data.bin(winrar/7zip/winzip)
plus
creating a new data.bin & it's making install impossible
see here what its doing - http://www.dc-unlocker.com/forum/showpost.php?p=29320&postcount=11
mutter mutter - still working on it