First try turning on headlights check to see they light up bright, this proves battery is not failing under load.
Try starting engine even if stater fails to turn and after to two attemps feel all connections from battery to starter motor, any that are hot are possible high resistance and need cleaning.
Then make a test lamp, to make one connect an indicator bulb with two leads, put one end on battery negative terminal, and then check bulb lights bright at soleniod. Then disconnect lead at starter motor, connect this lead to test lamp and try to start motor, light should come on bright. this proves voltage and current is reaching starter motor.
It could still be a poor earth, so disconnect lead from negative lead from battery and connect to body of starter motor try to start again bulb should light bright.
Hope the above helps, if you fit croc clips to test lamp you can use as an emergency torch for future, when you breakdown.
