Hi, Clive
It's high time you learnt Polish.
![Wink [;)]](/emoticons/emotion-5.gif)
But seriously, my English isn't very good so it was easier for me to write it in Polish.
Generally it can be said that:
If you give the date you use Past Simple
If things happened in the past but you don't know exactly
when or it doesn't make any difference
when, you can use Present Perfect.
The Present Perfect is a present tense. It always tells us something about now. If you want to emphasize the connection with NOW you use Present Perfect. If you don't want, use Past Simple.
Do not use the present perfect if there is no connection with the present (for example things that happened a long time ago)
I lost my wallet. (I lost it in the past. We don't know if he/she has it now)
I have lost my wallet. (I don't have it now and for example I can't by a present for my girlfriend)
Besides, Past Simple is often used instead of Presnet Perfect in AmE.
Regards
Dominik