Agreed - that is a timber lintel and runs the whole way across the top of the window opening. It holds the wall above the window up, so you can't just rip it out. But if it is rotten it may need to come out and be replaced (ideally with a steel or concrete one unless the building is grade 1 listed, or the rendering is being stripped off and the lintel will become visible). This is a more major job which will involve supporting the wall above the window with either Acrow props and strongboys or Acrows and needles
The wood in the third picture is wedges which are generally hammered into gaps in the mortar to carry something or other, e.g. a light or house name board, etc. They can come out and the resulting gap(s) can be filled with (lime) mortar. You can also see what appears to be a cut nail in the wall just above the timber wedges, which can also come out.
Looks like a slate building. Is it?