It's finally finished, I dug out the soil at the rear of the wall and installed a french drain to direct the water away. The rear of the wall was also sealed with bitumen and a waterproof membrane which has made it a lot drier - no wet mortar is noticeable at all now.
If anyone can recommend a slate sealer that gives a 'wet look' then please let me know
If anyone can recommend a slate sealer that gives a 'wet look' then please let me know