This web page about the recording "Celtic Sessions" by William Coulter has the following information on the tune:

"Neal Hellman learned The Heroes of St. Valery after hearing Martin Carthy play it in concert in Santa Cruz. Neal then taught it to me and we have been performing it together ever since. It is a retreat march, written by Pipe Major Donald MacLean from Lewis. He was a member of the 51st Highland Division, part of the British Expeditionary Force sent to France in 1939. Although the Division fought with great gallantry against the German offensive it was eventually forced to surrender at Saint-Valery-en-Caux. In 1944 after the Allies landed in Normandy, the 51st Highland Division was allowed the satisfaction of liberating Saint-Valery. The tune was first played at the Highland Brigade gathering in Edinburgh in 1947."

The tune is one Martin Carthy does in his video "British Fingerstyle Guitar"

At Iain Laird's webpage, in the Scottish section, there is history of the 51st Highland Division at St Valery.
NOTE: there is a .wav file of the march played by a pipe band that takes a (very) long time to load. This page states that the tune was composed based on a folk song Pipe Major MacLean heard.