You need to dig out and 're point with lime mortar as this will allow the property to breath.It also stops dampness spreading.You can get lime mortar at most builder merchants although a little dearer and don't apply unless the temperature is generally 5 degrees or higher.

Without actually seen the job in the flesh i would say leave well alone. You could do some serious damage attempting to rake out and repoint. Repointing with hydraulic lime mortar is not the be all and end all for repointing brickwork. Over the years i have been instructed to repoint brickwork using Hydrated Lime and cement to the proportions of 5 parts soft sand 1 part cement i part hydrated lime. This mix is also used for laying bricks. I have not experienced any problems/comebacks using either. Good luck.

Most buildings built before 1920 were typically built using lime mortar. Unfortunately once a building has been repointed in sand and cement, it is very difficult to remove without damaging the stone or brickwork.

If you're lucky, it was repointed using a weak cement mix (6:1) and can be safely removed. If not, then there isn't much you can do except angle grinder the mortar back out. This method is far from ideal.

Depending on the age and condition of the masonary of your building, different types of lime and lime mixes can be used. NHL lime has been the "go to" product for a while now but is increasingly becoming less popular as they cure very hard which can also damage masonary work. Hot and fat limes are becoming recognised as the correct product to use.

Try to think of the lime mortar joint as being the perishable part of exterior of your building. It will absorb most of the moisture qhile also allowing it to evaporate. It will also perish before any bricks and stone hence the saying "perishable part".

UNDER NO CIRCUMSTANCES SHOULD CEMENTOUS PRODUCTS OR MIXES IN ANY AMOUNT BE USED ON PERIOD BUILDINGS!!!!!