Curious. What is your wall construction, and how long since it was done without the dampness reappearing? DPC is often injected at the same time as other repair work is done, and the wall is dried out, and even if the source of the dampness is still there it might take years for the dampness to reappear if walls have been re-rendered on the inside and woodwork replaced etc.
There is obviously a theory, and maybe some lab testing that backs up injected DPC, but these presume that all period houses are constructed in the same way with repeatable materials, and that kind of misses the point that old houses are extremely variable. Injected DPC are apparently more likely to 'work' on some types of stone and not so well on others, but since the internal structure of many old houses is not known, I don't see how it can be any more than a hit or miss.
The older houses I have lived in are made of walls with variable thickness of walls, with variable materials ranging from whinstone to sandstone, and variable infills which could be made of anything that happened to be close to hand, which could be mud, clay, mortar, stone etc. I really question how the installer can know whether they are injecting into the outer wall, the inner wall, or the rubble infill, and if there are any gaps in the damp proof layer then the water will continue to rise at that point(s).
In general I still don't see a very strong argument for using it. If there is so much water in the base of a wall that an injected damp proof course is needed, then it will continue to build up below the DPC and eventually find a way through, or it will just saturate the stone footings. My understanding is that some stones can be weakened by saturation, but I don't know enough about it to know the real impact of that, but personally I'd prefer to find the source of the water rather than stick a plaster over it.