Always use the time zone that matches that of London. From the last Sunday of March till the first Sunday of November, Nigeria & London have the same time zone which is UTC +1 due to daylight savings time coming into effect. So this makes it easier to know when each session is starting. That is why @yokoyi used UTC+1 time zone.
But, outside the periods I just mentioned above, London time will fall behind Nigerian time by 1 hour (due to daylight savings time coming to an end), so you in Nigeria should simply add 1 hour to the session open and close times.