I have a copy of Christmas Truce by Malcolm Brown and Shirley Seaton.
It is about Xmas 14 but the final chapter is about Xmas 15 and there is a postscripts chapter about later events not always at Xmas.

In 1915 strict orders were issued on both sides against fraternisation, and harassing fire was used by both sides to deter it.
All the incidents described are of artillery fire on opposite lines and no man's land. There were casualties on both sides.

The incident mentioned is unusual and probably unique. The machine gun fire would have been indirect and fired from well back, but anyone exposed in NML would have felt fired on. A machine gun firing at close range directly at exposed troops would have killed people.