Window measurements for DIY install

[alan888]

I'm going to attempt to install my first UPVC window, replacing an existing very old UPVC window. It'll be certified by building control.

I know I need to measure from the outside and its brick to brick. The width is simple, brick to brick.

But the height is a little difficult to measure. The top of the window externally appears to have a lot of (what I think is) mortar and possibly a 0.5mm steel lintel (but not sure). I've included a photo of the top of the window from outside and inside. I think the window there now is a bit short and the new window can be higher up to fill the void if I remove the mortar that's there.

The inside has a trim at the top of window and if I remove this you can see a gap and the other side of the mortar (?).

Does anyone know what that is above the existing UPVC window?
Is it just mortar that I can remove?
Is the thin strip above the mortar a steel lintel?

 

[God]

From the inside it looks like a concrete lintel but from the outside you can see a steel lintel. Just measure the height of the existing window. cant go wrong then.

 

[alan888]

A concrete lintel inside makes sense, explains why it looks neat/straight from the inside. But agreed, I'll order the same size window. Thanks!