hello mr threepwood
have you enabled Use external Play/stop on the ableton link panel or have you enabled it on the midi panel ? Make sure it is on the ableton link one.
Also, make sure it is enabled on ableton live as well, when you choose midi controllers, there should be some buttons or checkboxes that read track/sync/remote/mpe , make sure all of them are checked
To stop sequences just use the active or the mute button on each unit
Let me know how it goes !