How can I rehook up the arm of my toilet cistern inside the tank?

I have a 'Dauntless duoflush' toilet cistern and the arm has become unhooked from where it connects underwater to whatever activates the flush.
For the life of me I can't see what to hook it back on to. There does not seem to be an obvious docking point.
At the water end of the arm there is a simple removable hooking device. To what does it attach, please?
A diagram would be appreciated.