Thanks guys, the Maxim route is very expensive here in South Africa, so as there doesn't seem to be an existing Arduino charger around, I have sketched one based on my existing design.
The advantage this has for me is that I can program the chip not to switch the charger back on again when the batteries have cooled down, which used to happen which keeps them topped up, but not needed most times.
I could have it recharge every couple of days if left on charge...
One of the thermistors touches the battery to sense the heat as the charging cycle is finished. There are 2 thermistors to cancel out any temperature changes in the unit. With a red LED and a 1 ohm resistor it will charge at about 750mA, so the 12v supply should be able to handle this, or increase the 1 ohm to 2.2 ohm to reduce the current to about 350mA.