"So instead of BPT spending $43m on a never never project, it...

"So instead of BPT spending $43m on a never never project, it will just have to spend an extra $25.8m to free carry SXY. I hope this is the case."

g12, I do like your novel interpretation of what a $43mill free carry entails but alas $43 mill is not $25.8 mill.
However the original Lattice commitment was for "up to $43 mill in investment ..." which might give you some leeway should you envisage several dry holes deterring both parties.

"Release Date: 16 April 2018 ​

Senex Energy Limited (Senex, ASX: SXY) today announced that it and Beach Energy Limited (ASX:BPT) had reached agreement to transfer up to $43 million free-carry commitment on the joint venture’s unconventional gas project to the Senex operated Cooper Basin western flank oil assets."