That's so flat and so out of sight that I'd consider a piece of 50mm x 6mm flat bar fitted under and painted dark red. If you make a basic template your steel stockist will be able to bend to shape.
If you deeply rake out the top half and repoint it first (it has dropped a bit on the left but it just adds character) you will be able to safely remove the lower part, pop the steel in and rebuild
Easy diy job. The hardest thing will be that scary lime mortar stuff.
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