You use the Present Perfect to say that an action happened at an unspecified time before now. The exact time is not important. You CANNOT use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc. You CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc. Examples: I have seen that movie twenty times. I think I have met him once before. and here some more examples http://www.englishpage.com/verbpage/presentperfect.html

Antti Potiomchin, Julia has given you a pretty good answer. You asked for an answer from an Englishman, which means a native of England. I am Canadian/American and the usage is a little different there. We tend to use the simple present in situations where a British native might prefer the perfect tense. In America the past simple is often used when you are speaking about a specific past event even if the time is not mentioned. So, in the sentence, You said that "blah, blah, blah,..." if you are thinking of a specific time in the recent past (when the person said something) the past simple with 'said' is appropriate. Its relevance to the present moment would be in the context of the conversation. This form is more often used in Canada in speaking. On the other hand, You have said that "blah, blah, blah,....." is also correct. This form is also used in Canada. It is a little more formal and could be understood as stressing the fact that he did, indeed, say it, or again, just to stress the relevance now of an action that happened at some indefinite time in the past. (Perhaps you don't remember exactly when your conversation partner said it, or exactly when he said it is not important to the conversation.) The use of present perfect or past simple is often just a matter of choice; it reveals the thinking of the speaker. Native speakers understand this intuitively because they learn the language by ear. The best way to pick up on the usage is to listen to native conversation and let your brain analyze it naturally, in the way a child does. You will have to wait for an answer from an authentic Englishman...there are a few around in Italki. :)

It honestly depends on what you want to say. "Englishmen" usually say whatever is appropriate for what they're talking about.

No more useful answers? I am really looking forward to seeing the most appropriate opinion. Am desirable from the native speaker.

Good answer Julia!