In the film 'Zulu' what song is sung by the soldiers to keep up moral, and why this song?

The song Men of Harlech is something of an unofficial anthem in Wales. Every Welsh person knows the tune and despite the variety of lyrics over the years, the martial air has become identified with the country's determination to retain its identity. Harlech Castle in north Wales, one of the 'iron ring' of castles intended to subdue Wales in medieval times, remains as a picturesqe reminder of the ultimate futility of the invader's ambition.

Outside of Wales, the song has become well known as a result of the film Zulu which told the story of a small detachment of soldiers and their epic stand against a huge Zulu army in southern Africa. The soldiers were from a regiment which recruited in south east Wales and the borders and their heroism came to be compared with the bold exploits of their ancestors in ancient days. The song would have been known in Wales before the Zulu War but was it actually sung at the battle of Rorke's Drift? The curator of the unit's regimental museum thinks it unlikely, since the song (first published in 1860) was only officially adopted by the regiment in 1881, whereas the action depicted in the film took place in 1879. Although we have the 1860 lyrics of the song, we do not have the version from the film. This was written especially for the film and enquirers are advised to seek the copyright holders (whoever they may be). http://www.data-wales.co.uk/harlech.htm

Private Owen (Ivor Emmanuel) also sings Deck the Halls with boughs of Holly momentarily when the Zulus appear to have retreated.