Water-damaged concrete window lintel

AMC174

Hello forumites, I'd like some advice please. A small (about 4 inches long, wedge-shaped) piece of concrete fell out of the sitting-room window lintel a few months ago. You can now see the (rusty) metal bar inside it. There are no cracks around it, and it appears that water from an occasionally overflowing first floor gutter above had penetrated the lintel over time and caused the metal inside to corrode and expand, pushing out a piece of the lintel (I think). We are wondering if a straightforward repair to the concrete would be ok - and how to go about it ie. what to use etc? Or if it's something that needs a more radical solution involving a builder? 

grumbler

Any repair will be purely decorative - like plastering a wall. I don't think that anybody here can tell you whether it needs replacing or not.