[BerntR]

To get the downstroke more from the inside - does it help to flatten the swing coming down or/and have the ball a little more back in the stance - or are there others things to work on that may help to get rid of this menace?

It is a little easier to hit the ball from the inside with ball back in stance. Key for hitting it from the inside is that hands are in front of the ball.

Further, it is a lot easier to learn to hit the ball from the inside with the ball back in the stance.

You also need a grip that is strong enough to match. So that the face squares by itself by the time of impact.