Just to update - transit (or even a quick turnaround) in IST is a breeze.

There are transit desks to the left of the immigration queues - one for Turkish and one for other airlines. You collect your boarding card, and then go up the escalator to the departure lounge. No need for a Visa if you are on an international transfer (although I believe you would for an internal - as stated above by Smirnoff you need £10 for a visa - get that before you join the lines at immigration from one of the Visa desks either side of the immigration desks). I was in departures 15 minutes after leaving the aircraft. Annoyingly BTI said the connection time (1 hour 25 minutes) for the later LHR departure wasn't possible ... I guess sometimes it isn't.

IST security checks are repeated at the gates - the metal detectors are set to make them very sensitive - 50% of passengers had to remove their shoes, lots of watches and one guy even had to remove his spectacles as they set the alarm off!