@Neil_McRae
As mentioned by @Shinedown78 most AC systems use IR remotes for control. They can be therefore controlled by Logitech Harmony which has integration with Smartthings although I am a little dubious about how much AC specific control this would expose.
All the various ‘smart’ systems for controlling AC systems that I am aware including Tado Smart AC, Sensibo, AirPatrol and AmbiClimate also use IR to control the AC system. Some of these can be used with Smartthings either via apps or IFTTT.
However another approach is specific for the Daikin range of AC systems. Many of these support a Daikin module for controlling their AC unit via their app over WiFi. There is a matching user contributed Smartthings app.
Since this is a direct link it would in theory be more reliable and allow some bi-directional reporting like its own temperature setting.
Disappointingly Daikin have not progressed their WiFi controller further to have official integration with any smart home platforms other than IFTTT. I would have liked to see them add HomeKit and official Smartthings support amongst others.