This is not an exhaustive or a complete subject. It is likely to change over time, with edits, additions and deletions, and is simply a collection of my thoughts based on many years of experience. You should follow your instincts, and you must always seek your own professional advice.
When rainwater runs down the outside of the downpipes it is probably indicating that the pipe is blocked. Although it is well-known that drains can become blocked, it seems to be less well-known that downpipes can become blocked with roots and vegetation too. It is quite easy to remove the pipe and clear the blockage. Do this as soon as you notice that there may be a blockage, to avoid the wall becoming saturated.
Disclaimer: Anything posted in this Blog is for general information only and it is not in any way intended to provide any advice, legal or otherwise, on any general or specific matter that you can rely on. You should always seek your own legal and surveying advice.