How long does maghrib last?

How long does maghrib last? For example here in the uk sunset is at 3:51. For whatever reason something delayed us would praying at say half 4 still be valid?

Maghrib lasts from sunset to the end of the evening twilight (i.e. until the time of the Isha athan). The time in which the maghrib prayer is valid is usually a little over an hour after the maghrib athan.