300 tdi spins starter with no key in ignition!

Hi, I have a 300 tdi that spins the starter with no key in ignition. Vehicle had been sat on the drive for several hours when we lifted the bonnet to adjust the idle. Without any warning the starter engaged and we had to pull the battery to stop it! As it was raining at the time we wondered if something got wet and shorted out but it was left overnight and is still doing it. Anyone got a suggestion where to start looking?? Cheers, John.

Opening the bonnet may have sent some water into the electrics. If you pull the starter wire off the solenoid and there's 12v on it, the next step is to look at the back of the ignition switch.

Cheers for that zeaphod, tried a gentle tap or two on the solenoid and cured it!

I would double check the wireing to the starter,just a bit of water alone would not do that. Y