pug 106 starting problem!!!

i have a p reg peugeot 106 and i am having intermitant starting problems with it. 90% of the time the car starts first time without any problem, but, on occasions, once i have entered the 4 digit security code and try to start the car i get no response. the ignition lights come on as usual and all features of the car work as normal but the engine wont even turn over. when this happens i have discovered i am able to start the car with a bump start and it drives as normal. the engine management light never comes on as a warning of any sort and the problem is not consistant to any type of weather, different types of driving or long or short journeys. i am a mechanic myself, but just cant work this out. any input would be gratefully recieved.