I can now see the DPC in a number of your pics, and it appears to be well above the ground - however, your ground levels appear to alter dramatically as you go around the house. So its possible that the DPC is bridged here and there.
Bridging is to be avoided, even in a cavity wall.
Bridging can also occur inside the cavity due to debris build-up. Is there any damp inside the house at skirting level?
And in a 1959 house of your type there would be no slate in the DPC.
The conservatory DPC is doing its job.
Wall tie inspection is best done from various points and different heights using a camera or boroscope in the cavity.
There's a brick with newish mortar low and left, near the ivy, where the only inspection on that wall was possibly made - post #5, pic2.
The brick was removed for the inspection.
However, if there is CWI then it could be blocking your air bricks inside the cavity. Have a look. It would also pay you to inspect the condition of the joists tails in the sub-area wall pockets.
The air bricks that I'm referring to are below floor level and allow through ventilation of all suspended floors.
You should have air bricks every 1.5m to 2m according to Regs.
The circle pattern doesn't fit the profile for CWI so I guess that someone somehow determined that thats where new wall ties needed fitting or replacing.
As you recognise, the deck and garden are probably bridging the DPC, and the wall is being soaked with splash near the hose bib.
At some stage you should open up the chimney breast flue and have the flue swept, and the stack examined for weathering. The redundant flue should be vented bottom & top.
Go and examine the chimney breast in the loft for leaks or minor structural movement.
You have a knock-through at the deck - is there a nib of brickwork supporting the right hand bearing of the lintel, or does the lintel go into the chimney breast cheek?
Yes the ground level does alter dramatically and we can't understand why as next doors garden doesn't yet the front and back line up with the pavements outside (we're on a corner plot.) The only way it would all be one level is to build up where the decking is and up to the step that stops half way down the conservatory but then it would be above the DPC.
As you can see from all the build up next to the front of the house/in front of the conservatory it will be hard to find any airbricks until it's all cleared. Any others will be round the back under the decking which will also be hard to find until its cleared.
We haven't noticed any signs of damp and skirting level. Where the walls being splashed by the hose isn't part of the main house it's not plastered or anything inside but has fitted cupboards, it's possible there could be damp in that wall but I haven't checked.
I will ask the OH to go and check the chimney breast. when we moved in there was the biggest spider on the wall that could be seen just from the opening about 5m away and neither of us have been back since! What would leaks and structural movement be - cracks again?
I'm really sorry... I wouldn't know how to look inside the cavity and I don't know what joists tails in sub-area wall pockets are.
I also don't know what a knock-through at the deck is or which lintel your referring to going into the chimney breast? Sorry!