The arch stones comprised 16 cut stones, 3 large stones with cut rebates to one side, 4 large stones to the other side ( the ‘second’ stone was actually 2 stones) and then the curved upper section forming the arch itself numbered 9 smaller stones. As mentioned in the previous posts, the crane was used to position the more massive side stones, but I was able to lift and place these smaller arch pieces directly off the scaffolding. I used a little lime mortar, allowing around 12mm for the joins when I measured and cut the formwork.
I left the formwork in place for a few days but in theory, it can be removed earlier if necessary, once the lime mortar has gone off sufficiently—though it is probably safest to leave it 24 hours at the least.
It may be worth stressing again here that should there be any chance of frost, then delay using any work involving mortar. There are preparatory additives available to guard against the mortar freezing before it has fully cured—but they are relatively expensive and not 100% foolproof. If you are forced to take a chance on the weather, then at least find some good insulating cover for the next few nights.