So what do u think , cheat or gamesmanship ? after watching the video's

A bit of both from my point of view.

His experience at the time and Lewis' inexperience were both factors.

If I remember correctly Lewis needed a toilet break during this or a previous match and received a bit of banter from the crowd that Manley had picked up on as Lewis didn't handle it very well, the young lad got quite flustered.
So Manley, knowing that the young Ade Lewis could be shaken on stage played the 'A' game to a tee, both flustering Lewis and making Lewis look at fault in one fell swoop.

However, the TV showed that Manley did indeed 'cheat' as he was clearly seen muttering something like 'big it up Ade, big it up' as Lewis was about to throw - clearly against the rules.
But in doing this he knew that Lewis' head would go and thus you could call it gamesmanship however I would call it cheating, but thats just how I remember it.

Also, just to add, look where Manley is now compared to Lewis.

Lewis champion of the World and Manley scratching about on the Pro tour (didnt even qualify!).
Manley may have won the battle but Lewis certainly won the war.

Yep I agree with Rebel.

There is a fine line between the two and Manley in that instance overstepped it.

Manley's a Legend!!! He knew what to do, when to do it and how to do it. End of the day darts players get MUCH, MUCH worse from the crowd and if you can't handle it, you're in the wrong game. ;)