BMW diesel 2.0l in Land rover, with funny piston -Any idea ?

[hotrubber007]

Hello , need help from some more brains

I've just re-newed the engine in my diesel land-rover , it's a 2002 Td thing with bmw power. The old one cacked out on me, started bellowing out smoke and when i removed the head number 3 piston had a melted piston , really smooth molten edges. It has no wear other, had only some 70.000 miles on the clock and i'm not the one to kill an engine. The reason for this post is simple, i don't wan't to kill the new one .
Now that would be really stupid. The wastegfate seems to work well ( at least it's moving freely - unlike the one on my izuzu powered elan turbo, and the only fault i've found changing over the ancilleries is a weeping waterpump - but plenty of water in the system.
Could i have inaventently ( shit spelling from a belgian ) put petrol in my diesel ? melting crown must be due to heat of some sort - right ?

cheers - from a fella with a potential expensive test drive ahead

[ridds]

Check the injector, sure sign of overfuelling.

[hotrubber007]

Absolute logic Ridds , i'm changing them straight away

would i have been able to see that shortly before the engine gave up the ghost ? ie black smoke ?

cheers for your help