Rumour has that the MOD made a muck up of the tendering process, realised that things wouldn't be in place for the final round to take place, (I don't know what those "things" were) and asked Serco to carry on for another 6 months.
Realising that they MOD by the balls, it is said that Serco said 18 months or nothing which the MOD are said to have accepted.
I understand that SERCO have a 25 year maintenance contract on the Apache and have suggested to the MOD that they also take the ATC contract on similar terms, rather worryingly, the MOD seem to be warming to the idea.