OK, you were all wrong ! It's a cashew, the bit that we call a cashew nut is the bit at the top (but it grows the other way up so that part is at at the bottom when it's on the tree). The cashew nut is actually not a nut at all but a seed and there is a long description about pseudofruits etc. to be found on WIKIPEDIA : http://en.wikipedia.org/wiki/Cashew
Thanks for all the suggestions !
