Put www.time.is in to your smart phone as a useful check. “Time.is” claims: “to be synchronized with an atomic clock – the most accurate time source in the world. The displayed time will normally have a precision of 0.02-0.10 seconds. The precision depends on your internet connection and how busy your computer is.”
Your finish time should be to the nearest second and should be based on an accurate watch or clock. The time required is BST in hh:mm:ss. Watches should be set to GPS time (but note that there can be a delay in the correct displayed time when first switched on) , radio transmitted time from Anthorn Radio Station in Cumbria used in clock and watch synchronisation, or the pips on FM radio, (not digital radio or digital TV).