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Thanks for the reply.
So removing the crappy, crumbling slates to look for the DPC could be a good start?
When you say stripping the plaster do you mean inside?
Only recently laid a nice oak floor and decorated - not the news I wanted! (but thanks very much regardless!) If there's any other, less invasive things to consider first then I'm all ears. Otherwise I guess I'll have to accept that we're going to make a mess...
Quincy-
So removing the crappy, crumbling slates to look for the DPC could be a good start?
I reckon. If it's been cemented on, though, it may damage the brickwork when you remove it.
Also worth nothing that some buildings of that ear have slate DPC (also pitch, sometime none).
Taking the plaster off would allow you to see the problem with less stuff in the way, and to see if it dries with the outside fixes.
If it didn't, that's when I'd be thinking about lifting the floor.
Was there no damp when you laid the floor? Do you know what's under there?
NotThamesWater
It's difficult to tell where the damp proof course is with that pebbledash render and whatever is happening with the black stuff / cement parging.
If the DPC is covered over, that could be a reason way for wetness to get in.
You'd think that the DPC is just below the line of the render, but if it's not, and it's at the level of the base of the parging, then that's quite possibly your problem cause. In which case, you'd need to lower the ground level
Digging a deeper French drain likely won't hurt (you'll hit brick footings soon enough, I'd guess. Stop when you do...).
The render itself could be a problem if it's cracked anywhere - It's lovely and waterproof, whereas your 100 year old bricks and mortar are porous, and it will stop the water evaporating.
Stripping the plaster & looking under the floor will give you more answers.