Further thoughts though:
Havrincourt actually appears on trench map 57c but square 15 on that map is located to the north of, and slightly to the west of, the village itself. So that might well be the '15d' part of the reference, with the 'd' indicating the quarter of that square at its bottom right (as explained in my first link above).
Taking that to be correct, and again using the information from my first link above, you need to start at the bottom left hand corner of that quarter square and go 95 yards East and 80 yards North (remembering that the dimensions of a quarter square are 500 yards by 500 yards).
That takes you to a point on the north side of what the British troops called 'George Street', approximately 100 yards to the west of 'Ryder Street'.
That's a point almost due west of the centre of Flesquieres and slightly to the east of a point midway between the Canal du Nord and what is now the D15 road, which (as close as I can get it) is here:
http://tinyurl.com/y73hywwj
All of that is derived from spending over an hour studying this:
http://digitalarchive.mcmaster.ca/islandora/object/macrepo%3A70646
and comparing it with Google's modern map.
(Use the + and - buttons or, easier, your scroll wheel to zoom in and out on the trench map. Click-and-drag to move the map about)