Thought I'd finish this of by showing how I cut the glass and put tied the casements.
This is all slightly arcane falling somewhere between witchcraft and legend as new single glazing is not at all common but the act is both slow and satisfying and the result pretty durable on the whole.
Any way the first pic shows tools required basically a sharpie, a straight stick, and a glass cutter.
Measure the opening then take 3 or4 mm of and aim to cut that size. This is to make sure it fits and isn't to tight. Mark the size on your glass. Now I was using old drawn glass I'd brought from Liverpool which was far from flat. But it's still simple to cut. The biggest issue is removing the paint/putty/scratched edge. Mainly because it's a narrow cut.(at least an inch) this means it's hard to get purchase for the break. I use a batten with a saw cut.
The next step is to mark the glass using a ruler and sharpie dots. Then get the glass cutter and line the wheel up with the mark. Then smartly score going forward and listening for an uninterrupted zipp. Just once. Trying to make it edge to edge. Then put a small rod of wood under the score and carefully put pressure on both sides. It should snap! With smaller pieces the rod isn't necessary tbh.