Insignia engine seized

[Shamek]

Hi to all
Please let me share with you my experience with insignia which started of with seized engine and some more problems after all.
Hope there will be someone who went through the similar situation and would be able to advise the next steps I should take.
This is a standard 2.0 cdti SRI model with 99k on clock.
Not long ago a low oil icon come up on the dashboard and couple minutes later the engine stopped running, as I was on motorway I couldn't stop immediately but I've managed to get to nearest lay bay.
AA diagnosed seized engine, towed me home and suggested to scrap the car. I read all about problems with oil seal in sump and I've replaced it.
To check if the engine is damaged I took all apart, but couldn't find any damage or excessive wear on cylinders.
Once all build up back to it's original state the engine was turning absolutely fine by hand, but after couple turns by starter its stops and it will not turn at all, even by hand.
Not sure what may caused this problem as all parts have been put back as per manuals.

Please share your comments and suggestions.
Thanks in advance