But I didn't see anything of the kind:
The vertical plastic piece (held in with two nails and a bit of plasterboar d adhesive overspill) looks to me like it it some sort of cavity closer and its profile is such that I can't see how any sort of straight cleat could pass through/behind it. I certainly can't see anything on the exposed block work.
Any ideas how this window is secured in place? Incidentally, the inside of the frame shows no obvious fixings passing through to anything either.
I will continue to hack away when I can find a moment, and will look to rem ove the window ledge also in case there's anything under there (I wouldn't expect there to be as it'd also require something along the top which doesn 't seem like a good place to secure a window what with the lintel above it) .