Moisture/damp under extension

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Hi all. Bit of a long winded one here apologies!

I am trying to identify exactly why this section of wall is damp:

The other side of this wall is the neighbours garden

Directly behind this wall there is also a rainwater gutter which is within a very tight gap, just about wide enough to walk into sideways due to neighbours conservatory and our wall

Everything right of this pillar is our extension which was built by the previous owner
We have taken up floor tiles as part of an ongoing renovation

The channel you can see is where the floor tiles began and were about 15mm higher than the main living room floor seen here on the left. This is concreted and is like this throughout the ground floor
This channel was created as there were more tiles under the floor tiles!

The channel is not wet but ever so slightly damp...you wouldn't be able to tell this but when I pick up the soil it smells damp if you know what I mean and shouldn't be like that. Again it is very minor but I know it is not 100% dry

As you can see there is some kind of DPC either side of the channel but not in the channel
I believe this is what the issue may be but I am worried about the drain outside also

Last year I squeezed into the gap outside and just put some concrete mix on to our wall and at the join between wall and floor. it's rendered and the render stops about 300mm before the floor. I was worried water was getting in

The internal wall seemed to stop getting worse but as you can see it is coming through new brand new plaster

I am thinking about concreting the channel and using a DPC also. However I will need to seek some independent advice but really just wanted to pop in here incase anyone can spot something obvious or provide initial comments to help
Thank you!
 

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May be a daft question but do you have a pressurised heating system and are the 2 pipes going roung the pillar for radiators. If yes does the boiler lose pressure and you have to top it up often. Hard to really tell from the pic. but I thought it might look as though it had a slight leak at the L.H. top elbow
 
May be a daft question but do you have a pressurised heating system and are the 2 pipes going roung the pillar for radiators. If yes does the boiler lose pressure and you have to top it up often. Hard to really tell from the pic. but I thought it might look as though it had a slight leak at the L.H. top elbow
Hi no leaks. The pic isnt the best. Just to note the grey tiles in the channel are removed on the opposite side. These are still in place temporarily.
 

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