S14 showed 159 ad-hoc charters, so far S15 shows 43. No doubt more will emerge for sporting events, diversions, etc.
The lack of English teams left in the UEFA and Europa Cups are not going to help add to these figures...
However the Rugby World Cup might boost some figures in September. Especially as Stadium:MK and Wembley are host venues and quite close by.
Im sure the airport will also be vying for some of the traffic for matches during the contest taking place at Twickenham and the Olympic Stadium too.
Rugby World Cup