Okay, I have done a major re-work of my driver board.
I am going to use the same LT3496 driver, but in boost configuration so that I can use a +12V input. This will also negate the need for a separate 5V power supply as the LT3496 can be run from 12V without issue.
I have also decided to switch to an I2C interface. I'm trying to find a chip like the ATTINY85, but runs from 12V. If that's not possible I can always power the chip from the Arduino as it takes <3mA at full load, but it would be much neater and easier to use a chip that can be run from the main +12V connector (that comes from a computer powersupply).