Hi everyone,
I'm posting here in case this is useful to anyone else. I'd been puzzled over whether I could 'use' Butler in the Trello app on my iPad and iPhone, and I couldn't see an answer - so I went and experimented. (I'm a relatively new user, with a Gold account).
The Butler power-up is available on the Mac OS (desktop) application. Once you've selected the power-up, you can create and edit rules etc. (I've made a couple of simple ones for recurring daily and weekly actions).
Once you've set up and activated your rule in the desktop application, it then works perfectly on both iOS applications.
Don't be put off if you read somewhere that Butler 'can't be used' on your iOS device. Although you cannot create, edit or switch on/off your Butler rules on your iOS device, once they're set up, they work perfectly.
David