ferris.. To add to what my learned friend WF said, everything at London is under radar control and ATC calculates EATs, etc. It's extremely difficult to provide accurate EATs due to so many variables, eg a go-around loses us a slot on the approach which adds a couple of minutes to each subsequent EAT. We try to provide accurate updates but this is not always possible. Pilots accept this and stay in the holds until instructed to leave. We can take a/c off the holds way bove the minimum stack levels if necessary. The "accuracy" of getting traffic off the hold at a particular time is not relevant as we can adjust track lengths within the intermediate approach quite easily..