The bowler's action

"I was playing in a match and the opening bowler didn't inform the umpire of his action. As a result on the release of the bowler's first delivery the umpire called a No ball.
One of the opposition players wasn't happy with this and advised it is the umpries responsiblity to find out the bowlers action and not the bowler to tell the umpire? I would be grateful if you could clear this up.

If bowler changes for round the wicket to over for example or changes hands he must inform the umpire of this change I believe? So why wouldn't the bowler need to inform the umpire of his action in the first instance?"

Law 16 (Start of the play .....) informs us that there are a number duties that the umpires must perform before calling Play at the start of the match and at the resumption of play after an interval or interruption. One of these duties at the start of the match is to enquire as to the opening bowler's intended action (mode of delivery) and then inform the striker before he asks for his guard. The intended action may be right or left handed, over or round the wicket.

It is the umpire's responsibility to ascertain a bowler's action when a bowler starts his spell.
It is the bowler's responsibility to tell the umpire if he wishes to change his action during that spell.

Law 24 (No ball) states that "it is unfair if the bowler fails to notify the umpire of a change in his mode of delivery. In this case the umpire shall call and signal No ball."