Oh yes, Alisdair, it is invasive, but, by the time I purchased it, I did not know that.
Perhaps, now, there are a lot of Aponogeton in the lake close to the Castle of Ermenonville, where my garden was settled. We were living just behind the Castle and I discovered, when I cleaned the property we bought in 1982, a charming old washtub. It was used 2 or 3 centuries ago by the peasants working for the castle owners. So, I planted many things in this place, without taking care about the consequences of the invasiveness.
We can make a lot of mistakes when we are inexperienced.