Hello everyone!
I am also currently scraping out joints on ours. The mason came and looked at it and then said that I should use B-mortar to regrout, partly because a lot is missing in some places and partly because the lime mortar that is there comes loose easily and dusts. Talked to the local building merchant who recommended C-mortar (lime mortar) because he said that B-mortar will not bond with the lime mortar and therefore won't sit properly, plus B-mortar is harder and will crack when the house moves, as it is not as flexible as C-mortar. I mentioned what the mason said, which the building guy thought was strange, so he called the mason.
It turned out that B-mortar is what should be used from a fire safety perspective, and we will want to make fires. Both of them then meant that it was the best but that I must scrape out 2 cm of the current joint for the new one to hold and to minimize the risk of it cracking. I will regrout and then wash with tile cleaner, as I have read here that someone did, and then brush with boiled linseed oil, as it binds. If you don’t want to regrout, you should be able to bind the joints with oil
