In the high latitudes in the north of Europe, North Asia and Canada the astronomical signs that signal the start of Ishaa’ and Fajr prayers do not occur for some days in the summertime. This happens because of the shallow depression of the sun below the horizon in this period, which is the reason for the bright summer nights you may have witnessed in Edinburgh. This lasts for three months, starting in May and ending in August.
There are a number of ways that scholars suggested to estimate prayer times when their signs disappear. The methodology used in the Mosque for this year is the opinion of the International Conference of Prayer Times in which the time of the prayer, when its sign disappears or becomes extreme, is estimated based on the length of one-third of the night in the same place.
Other mosques may adopt other methodologies, like fixing Fajr time for the full abnormal period around 1:33 am which is also an acceptable approach. The important thing to remember is to stick to one approach for the full summer. Allah knows best.