I installed a new starter motor yesterday (bought new for $220 which is alot better than the $1000 quoted by the dealership) and the car starts and runs perfectly. Not sure what the original cause was - an intermittent fault in the starter motor which eventually failed or maybe an intermittent fault in the ignition switch which is yet to return. I'll just have to wait and see. I learned that the problem definitely wasn't the immobiliser which is showing the "key recognised" symbol in the cluster when I insert the key.