To ensure the wall doesn't fall down, a hole is knocked through the stonework, so that the support of an acro prop can be inserted. After the stonework below the support is dismantled, the old lintel can be replaced and the stonework can then be re-built using hydraulic lime/sand mortar. The new doors shown in the photographs below were made and fitted by Andrew French Carpentry, based in North Devon http://www.af-carpentry.co.uk/ - really beautiful work, thanks Andy.
Click on the first image to scroll through.