My son has just bought a terraced house 1888 era.
Had chemical treatment to mortar course, needed (EU regs) to strip back to
brick 1 meter high.
Professional job - drill out and inject. Re-plaster with special damp
Guarantee given for 20 years!
Need to consider whether any bridging of previous damp proof substrate.
Heard of rubble at back of closed fireplace for example.