Further to the last post, I had a brief conversation with a blacksmith (who happens to be a dab hand at period property renovation) and who assured me of the structural virtues of brick arches. And that, if I could support the arch whilst doing it, adding proper supports both sides would give more than enough structure to safely resurrect the arch without need for an iron bar.
So I sat with a cup of tea and planned how to attack the job, and in what order things had to be done to stop the thing collapsing.
The arch bricks are deep, and supported by two separate oak lintels. I was able to cut away the larger of the two lintels and remove the fill above it to expose what I was working with properly. The large protruding stone on the left is huge, and solidly embedded in the wall, so it gives a good platform for building another layer of stone on top and the arch off that.
On the right, the oak has been cut and left in place (it took a hell of an effort with a chainsaw!) and will form the basis for a platform on the right hand side.
Once cleaned up of all the old lime and bits of fill, the protruding stone shows a really nice flat face for mounting another stone on top of, and by using an oak wedge, and bits of stone fill, I was able to balance a perfectly shaped (and interesting) stone on top of this, lock it in to place and then use NHL5 mix to secure it in place.
I had to cut the second oak lintel in order to get the new stone in to place. It's enough of a cantilever to hold the remaining fill and brick arch up until the NHL cures (dry in 2 days, cured in 7-10).
Once it's cured, I'll build a wooden former and add another layer of bricks to form a lower arch - the aim being to match the height of the outer arch - which you can see in the above photo is about a course of bricks lower than the inner arch.
The new head height of the doorway, with a raised floor to minimise the issues with our lack of foundations, will be around 6ft at the middle of the arch. That's a couple of inches higher than some of our internal doors!