Success!

Hurray, the whole thing went much more smoothly than I could have hoped. The notaire explained in stilted English that the septic tank, although it didn't conform to current standards, works fine and would continue to do so for several years and no enforcement would be seen of the clause that it should be replaced within 12 months. I had already explained this to the buyer, but naturally the notaire repeating it carries a whole lot more authority.

So yip yip yipee! Weight lifted. Ulcer gone.