Check the starter solenoid.
If it is starting as soon as you attach jumper leads then it sounds like the started solenoid is jammed/fried.
This could also cause the battery to run flat because the starter motor would be running constantly.
I'm not familiar with the GS650 but starter solenoids are pretty generic items on most bikes and I'd be reasonably confident you could find something that would substitute - even in UB.
If it is only jammed its even possible a few squirts of WD40 etc. might free it up.
Garry from Oz - powered by Burgman