In the case of cities such as Edinburgh, which are situated at a high latitude where signs of nighttime disappear (and the two twilights unite) for around 3 months of the years, mosques must use a method to estimate the time for dawn and dusk which affect the time for fajr and ‘ishaa prayer. There are multiple accepted methods to make these estimations.
To make it systematic and easy upon yourself, the recommended act is to pick any one of the accepted methods and follow it throughout the summer period, and especially so throughout the month of Ramadan.
However, it is also acceptable to combine maghrib with ‘ishaa some nights, and then not in other nights, based on how much of a hardship the late ‘ishaa time might cause you. It is also permissible to pray some nights in a congregation with one mosque, even if you have decided you are following the timetable (and thus method of calculation) of another Mosque where there might be a difference in those ‘ishaa or ‘fajr times.