Well, the following is the relevant extract from the rules pertaining to the auction, copied from Colin Cooke's website:-
The emboldened bit is the most relevant in this regard.
So essentially, if you made a bid at 11.59pm on 23rd May, the auction for the item you made the bid on would not close one minute later at midnight, unless you were the sole bidder. If there were other bidders, they would be notified of your late bid, and offered the chance to raise their bid. Obviously, if they declined, the item is yours. If one or more raised their bids above yours, then you and the others, in turn would be re-notified of that higher bid. This process would continue until such time as all but one of the bidders dropped out.
Not sure how long would be given between your last competing high bid and the coin being yours, if there were no further bids.
That's how I understand it anyway, Jon. Hope it makes sense.