Well, after some more thought we thought of a way to engineer a solution to the possible problem of having to divert the water from the sedum roof downpipes should “organic blooming” be an issue. Yes, as all boys will spot it is just like a railway siding that you might get after Christmas to augment your oval loop of track! An Osma 45 degree Y base with……..
one of these, something I did not know existed, but is called a “bung” (something I thought you gave a concierge) to block up the “siding”.
In the event that we have to divert the flown of the down pipes away from the tank, we just take the bung out of the “siding” and put it in the main flow to the tank (the other branch of the “Y”).
So now HMS Overflow has a second conning tower plus a side mounted torpedo tube (according to the children that is) and we can backfill and cover it all up.