Here it is, in all its glory!
And, never forget that it is incorporated in the Union Flag to show that the Union between Scotland and England, from 1 May 1707, is indissoluble! The white saltire, bordered by blue is the St Andrew's Flag superimposed on the Flag of St George, for England, and the red saltire superimposed on the white one, is Ireland's St Patrick's Cross.
From the article on Wikipedia on St Andrew and Scotland:
Several legends state that the relics of Andrew were brought by divine guidance from Constantinople to the place where the modern Scottish town of St Andrews stands today (Gaelic, Cill Rìmhinn). The oldest surviving manuscripts are two: one is among the manuscripts collected by Jean-Baptiste Colbert and willed to Louis XIV of France, now in the Bibliothèque Nationale, Paris, the other is the Harleian Mss in the British Library, London. They state that the relics of Andrew were brought by one Regulus to the Pictish king Óengus mac Fergusa (729–761). The only historical Regulus (Riagail or Rule) whose name is preserved in the tower of St Rule was an Irish monk expelled from Ireland with Saint Columba; his dates, however, are c 573 – 600. There are good reasons for supposing that the relics were originally in the collection of Acca, bishop of Hexham, who took them into Pictish country when he was driven from Hexham (c. 732), and founded a see, not, according to tradition, in Galloway, but on the site of St Andrews.
According to legend, in 832 AD, Óengus II led an army of Picts and Scots into battle against the Angles, led by Æthelstan, near modern-day Athelstaneford, East Lothian. The legend states that he was heavily outnumbered and hence whilst engaged in prayer on the eve of battle, Óengus vowed that if granted victory he would appoint Saint Andrew as the Patron Saint of Scotland. On the morning of battle white clouds forming an X shape in the sky were said to have appeared. Óengus and his combined force, emboldened by this apparent divine intervention, took to the field and despite being inferior in numbers were victorious. Having interpreted the cloud phenomenon as representing the crux decussata upon which Saint Andrew was crucified, Óengus honoured his pre-battle pledge and duly appointed Saint Andrew as the Patron Saint of Scotland. The white saltire set against a celestial blue background is said to have been adopted as the design of the flag of Scotland on the basis of this legend. However, there is evidence that Andrew was venerated in Scotland before this.
Andrew's connection with Scotland may have been reinforced following the Synod of Whitby, when the Celtic Church felt that Columba had been "outranked" by Peter and that Peter's brother would make a higher ranking patron. The 1320 Declaration of Arbroath cites Scotland's conversion to Christianity by Andrew, "the first to be an Apostle". Numerous parish churches in the Church of Scotland and congregations of other Christian churches in Scotland are named after Andrew. The national church of the Scottish people in Rome, Sant'Andrea degli Scozzesi is dedicated to Saint Andrew.
A local superstition uses the cross of Saint Andrew as a hex sign on the fireplaces in northern England and Scotland to prevent witches from flying down the chimney and entering the house to do mischief. By placing the Saint Andrew's cross on one of the fireplace posts or lintels, witches are prevented from entering through this opening. In this case, it is similar to the use of a witch ball, although the cross will actively prevent witches from entering, and the witch ball will passively delay or entice the witch, and perhaps entrap it.
May St Andrew, Apostle and Martyr, First Called with his Brother, St Peter, pray for his people of Scotland, to preserves them from treason, and may he plead before the High Seat of Heaven that GOD SAVE THE QUEEN!