What I find odd is this picture:
I find it odd as it would indicate to me that your DPC is working as the wall only gets damp from 1.2m upwards...so seeing that I would be wondering if water is coming down the wall then being stopped by your DPC? Is it a continuous damp or does it get noticeably worse in wet weather or any other time?
If you have an old style boiler with a gravity fed system with header tanks etc. up in the loft it may be worth checking that none of the ballcocks are continuously slow'drip filling their tank - if they are it could indicate a pinhole leak in your pipework?? (although why it would go near what was your external wall is another matter but having a house of the same age I wouldn't discount anything without checking it!)