VW Caravelle 2.5TDi 2001...Occasional Non-Starter?

When I put the key into the Ignition and turn it, nothing happens. No click, no cranking, nothing! All the dash lights come on. This has happened twice, with at least a month between with no problems at all. One guy got it started after spraying WD40 onto battery terminals, and the next time a mechanic started it by pushing the vehicle a bit. He said it was an Ignition problem (not Battery, and no mention of Starter Motor). If this is so, what exactly is wrong? Does the ignition switch need replacing as a whole, or maybe only the electrical terminal on the end? Could it be the solinoid wire on the starter motor that's faulty? Any advice much appreciated, including what course of action to try first. Thanks.