Upvc windows installation to replace wooden frames

Hi All.

Have a couple days off coming up in a fortnight and wanted to swap a few rotten old wooden framed windows for upvc.

It's an old and quirky house, so I'd really appreciate your advice.

Wall 1:

I currently have old wooden window frames sitting in the middle of the reveal. Which makes me wonder, just how the frame can sit in what I'd expect to be the wall cavity.

I suspect each frame's bottom is supported by the sills, while the rest of the frame is held by mortar around the frame perimeter. Sounds dodgy.

Also, the sill sounds like a potential cold & moisture bridge across the cavity.

Wall measures 290mm thick, with 125mm internal reveal, 65mm frame thickness and 100 external reveal.

Imperial bricks. Mk1 eyeball sizes them at about 9" long, 4 3/8" deep and 3" tall.

All pvc window installation videos I've seen so far suggest installing the frames in the outer leaf by screwing directly through the frame. I am aware of brackets/lugs that can be used to secure the frame, but direct fixing to the outer leaf seems like a more straightforward option?

I’ve chipped away at the frame in one corner to investigate and there’s definitely a cavity with some slightly loose mortar – not loose enough to pull it out, but it definitely moves when poked.

I'm interested to hear your thoughts about the possible solution as to how to position the new upvc frame.

My initial thought is to use a cavity closer where possible, install the new window in the outer leaf, then plaster & decorate the internal reveal (which is going to become a lot deeper). Is that a sensible approach?

Now, the structural question: Since the windows seem to sit in the cavity, they are unlikely to be structural... Not sure if there's a lintel over the window - there's got to be, right?

One of them looks somewhat cracked.

Thank you.