I had a similar intermittent fault with my 1100 many years back.
It turned out to be bad wiring around the main starter relay. Not the relay itself, but the wiring at the back of the plug-in relay holder had become corroded and the corrosion had crept up the copper inside the cable in the harness.
Another place to look on the St1300 I discovered when perusing the wiring diagram for something else - that Bank angle sensor provides a signal for the bank angle relay. Any fault in that little circuit and according to the circuit diagram, and most of the system goes dead. Don't ask me where they are on the bike. I'd need to study the fiches.