Should Mercedes-Benz fix a corroded control unit under warranty?

My son bought a 2012 Mercedes-Benz C-Class from Mercedes-Benz last year, which came with a one year approved warranty. The warranty is just about to run out and it failed its MoT because of a nearside headlamp failure. It transpires that the problem was a control unit beneath the lamp being corroded. Mercedes-Benz have said that corrosion is an exclusion so they refuse to honour a claim and subsequently charged approx £1100 to fix it. The Mercedes-Benz garage could not explain how the water got there and there was no damage to the headlight. Do you think this is a reasonable stance by such a reputable company?
I think that is disgraceful and that he can take this to Small Claims and will probably get a ruling in his favour:
Answered by Honest John on

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