Yep accountant - another job i would be no good at ! ! Get a bit dislecic with numbers, always dialing the wrong ones haha. . .
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Possible faulty injector. Not atomising correctly meaning one cylinder running richer so hotter. No effects on engine temp as its only 1 cylinder. Extra heat from the faulty injector can cause the cracking and melting.
Only my theory though.
Hindsight now but did you ever run any kind of injector cleaner