Stopp The Climb, I am sure my friends and colleagues at Scottish control will correct any inaccuracies here but this is my understanding for the reasons for the delays in the morning.
A resectorisation Of the Talla sector has taken place, such that at the busiest times of day a new sector called Talla North opens up. Unfortunately there were no new frequencies available for this new sector to utilise and so instead they use a frequency that is normally used to split the Dean Cross high sector.
The Talla North sector is considered the busier of the two during the morning rush and so gets priority over use of the frequency. However Dean Cross is almost as busy dealing with the Southbound flow and so Scottish Control impose Minimum Departure Intervals (MDI's) between successive departures from Edinburgh via the Galloway sector. These are usually of the order of 1 a/c every 4 minutes. These MDI's take priority over any slot time that your flight might hold but does not mean that the slot can be ignored. Therefore if the MDI restriction puts your flight outside of the normal slot tolerance then a new slot must be negotiated.
What with this, remote holding, restrictive pushback procedures and trying to work out whether it is more expeditious to get a Talla departure away between 2 Galloways, all within minutes of first arriving on shift, if you listen very carefully to the frequency you can just about hear the ground controllers brains trickling out of their ears!