Im not really following this thread but all this talk of the roof is for what?
You build it like a house with a cavity wall or with some kind of membrane to stop the moisture getting through. Otherwise you're in the lap of the gods. If you specified to your builder from the outset that the building must be 100% watertight then you have some redress, though any builder worth their salt should have pointed out that unless you take the measures just mentioned then it cannot be guaranteed to be watertight and he should have made you aware of that, if he didn't then he's an idiot. Some water running down the outside face of the wall is a red herring that happens on every single external wall that gets rained on no matter what's it's built out of.
I'm pretty sure we said it verbally but can't see anything on the quote. However as I've said earlier from a "customer" angle I think 95% of customers would consider it a reasonable expectation for a building to be watertight unless advised otherwise, as we simply don't have enough expertise to think otherwise (obvious exceptions including no / wrong level for DPC, anything which allows water to sit against the building etc). I've had single skin garages which have been fine.
I guess the answer is - what next? Can you retrofit a membrane or is that what the suggestion on "dry lay" concrete up the walls was? It would only be necessary along the 4 courses of engineering bricks at the bottom as the rest is rendered. Or is Storm Sure on the outside a decent answer? I'll talk through with the builder but don't want to be fobbed off with something that might last a couple of years or make things worse!
Even if I can't get the builder to take some responsibility ,if it's worth it I don't mind paying a bit extra to get something I can just forget and not worry about, at least for a few years e.g. if Storm sure needs reapplying. This is short of knocking it down and starting again, the leaks aren't serious enough for that!
The roof still needs fixing! And I'm convinced there's an issue with some of the joints because the leaks don't happen everywhere (in the areas I've tested)