Wil not start after long journey

I have had a similar problem with a vauxhall astra,In this case the starter motor would engage and not disengage causing a battery drain there was a smell of electrical burning,the cause was the ignition switch which is basically made out of plastic , and also the mechnical side of the ignition lock was sticking and required lubricating this may explain why your battery/alternator tested okay.you can test for this by placing a test light on to the ignition feed of the starter motor[ the small wire ] and ensure it goes out when the engine is running and, move your key if there is play it will bring on the light.hope this is of help