"Daft" might be an unreasonable assessment, but you are missing the part where the mines aren't placed until the end of the activation phase. This means that all ships have finished moving for the turn. It is not currently possible for a ship to overlap the warheads on the turn they're dropped, because no ships can move during the combat phase.
Your right I am missing that bit! got caught up in the next turn bit and failed to read the start text Doh! Cheers Guys