I was on one of the packed Overground trains on Monday evening and even at Highbury & Islington, the train was packed with the air con packing up as soon as every carriage is rammed with passengers, which doesn't allow the cool air to flow down the train.

If London Bridge services are stopped at periods due to the ongoing work at the station for Thameslink, I'd be concerned about this even more, although I understand LO are adding a 5th carriage to all of their services on our line from December 2014.