What's the best way to fill this reliably?

[Driller]

Metal basement door and stone arched surround.

Plaster? Ciment with latex primer?

What would work best?

[Driller]

What I mean is that by just filling it with cement, as it's such a small amount surely after a while it will start to crack and detach itself? (This is the principle with teeth anyway ) Also, the metal frame is surely not very retentive/supportive which can't help matters.

What looks like rendering is from the 50s where they butchered the place to turn it into an institution. We put it back the way it was supposed to be but there a few left over things like this.

[Driller]

Brilliant tip thanks for that. I had a feeling yhe frame woukd cause problems.

[Driller]

Yeah thought of expanding foam but what can you finish over the top of that with? Resistance over time is a worry too.

In the middle the gap is 100mm high.

[Driller]

Not entirely sure what I want to achieve? Isn't it patently obvious I want fill the bloody hole in?!

Your mesh idea is interesting, that may avoid cracking I guess.