Before I launch into my query I'll list a number of apologies

1) Length of post. I would rather try and give you all the information in one go rather than drip feed.
2) Lack of any technical terms - I ain't no builder/joiner

Following some wild rain I have noticed some water ingress in my shed.
The shed itself is circa 6 years old and is treated annually with Premier Coatings Wood Preservative. There are no immediate holes or signs of damage to the exterior of the shed.
The shed sits on wooden battons which sit on garden slabs. There is no surrounding vegitation.
To ellaborate on the issue I've attached some photos.
I would also expand on the photos as folllows:
Photo 1: On scraping away some lose stones there doesn't appear to be any drainage. On this basis I thought that the water might be getting soaked up from the ground via the batttons.
Photo 2: This shows that the water ingress appears to be from higher up the shed wall. I believe it is "Tounge and groove"

Photo 3: This is to show the extent of the distance the water has travelled either being soaked up or running down.
Photo 4: A close up of the holes that I think might be part of the problem. Another apology here as despite numerous attempts I could not get this orientated the correct way

I've toyed with the idea of installing shed guttering to try an limit the splashback issue but given that this side of the shed is the only side exposed to the elements, I'm not overly convinced any guttering would help.
The other thought I had was to fill the nail holes with some sort of waterproof sealant. I'm sure I read a silicon based sealant would be best.
Other than that I'm out.
Any thoughts, comments, advice or possible solutions would be gratefully accepted

Thank you all for taking the time to read and comment.