After Theresa May's so called Chequers plan failed to impress, the Prime Minister faced another problem within her own party. A group of eurosceptics were gaining interest, members and momentum. The European Research Group (ERG), or the Spartans as they came to be known, had previously been a small organisation on the fringe of politics. In 2018, they fought their way to the front line. As Mark Mardell follows the twists and turns of Britain's 45 year relationship with the European Union, Mrs May must meet her newest challengers. Email: worldatone@bbc.co.uk. Twitter: #BBCWATO