That should have been taken into consideration, when built. But thats what i mean about high ground levels.

Over the years, people tend to add new layers of gravel/soil etc on top of the old stuff, and now the ground levels are higher than what they should be against the building.

The dpc is generally 150mm above external ground levels. External ground level, depending on floor makeup, being 150mm at least below your finished floor height.

Assuming its cavity walls due to council house,cavity going down 225mm from your dpc (depending on floor makeup), the cavity should stop anything crossing over , but again, sometimes (usually when initially built) the cavity gets filled slightly with mortar/rubbish from the brickwork above being built, and you dont have a cavity, you have a damp bridge letting water soak across.

Combine that with your garden running water towards the house, and its a potential problem. Ground might not be high though, and just the dpc work alone, possible also some drainage along the house at the bottom of the slope might fix it.

I dont know enough about the old damp courses to comment on it being worn out. I think plastic dpc started to get used in the 1970's, so its possible yours is slate or some other material.

Too many variables/possibles to really say over the internet. It might never dry out if the issue causing the damp isnt sorted, or you might be able to dry it out in good weather, but the next wet weather you get dampness again.

Could be high ground levels outside,causing damp to travel across especially if solid walls. If the specialist has recommended injected dpcs, high ground levels are the first thing id be checking.

But really, the specialist you've had out should be able to identify anything like that, and if you have any doubt, it may be worth a second opinion.

The injected damp course is something that splits opinions greatly. Some people say its great/bad. Others say rising damp is a myth entirely.

But at the end of the day, there is likely a reason for the damp in the first place, and that should be identified first, if the chimney fix hasnt been sucessfull.

If you've stayed there for a while and its only just started 4-6 weeks ago, i would think its likely something has happened to cause the damp.

Potential roof/flashing leak letting water into the chimney, leaky gutter/downpipe etc.

Waterway.

As others said, looks like you have adhesive around the spacer, which prevents it breaking where it should.

Havent used premix concrete bags before, but id assume do it much the same as mixing your own stuff. Wouldnt faff about trying to spray anything, personally.

Dont just dump it all in at once. Set it spinning, add some water, then add some of your mix. Let it mix in a bit before adding more mix and water.

I usually keep it fairly wet with the small mixers, only letting it get to your desired consistency near the end. Helps stop it sticking if its wetter while mixing. Make sure you can dry it up enough before full though.

The little mixers are bad for stuff sticking at the back though, no matter what. You can bash the drum with a hammer on the outside to dislodge it. Dont let the mixer owner see you though!

Other thing to watch, if you are tiling the full wall, is whether the remaining plasterboard area can take the weight. Depends on the weight of your tile (especially if porcelain) and your adhesive, but you can work it out,may require a bit of googling though

Plasterboard is on the lower end of weight capacity if i remember right. Tile backerboards higher.