Posted By: jemhayward

If you put flags on top of the french drain you will defeat some of the point of a french drain

We probably have no choice on that one, as it isn't our garden, so we'd have to put it back as it was, but the garden does slope towards our house so I suspect the main issue is't water from our roof, but water from their garden.

Another purpose of gravel rather than hard flat surfaces adjacent to the house is to minimise splashback of rainwater onto the walls. That can make quite a difference to how wet the bottom of the walls get. If you use sharp gravel you can also make the surface less attractive to rodents. As you say, it will depend on your neighbours' attitude.

PS To all, it's traditional to capitalise people's names, such as that of Henry French.
More detail in https://en.wikipedia.org/wiki/French_drain