Ah, now the link works for me. LOL, until I read the text I was wondering why she looks so Bavarian on the photo. Yes, I suppose it´s what you already have said, Ivy tripping, falling or walking a distance would require a double if Jane Freeman has walking difficulties. In one episode I noticed her ankles looked swollen, it may have been The Secret Birthday of Norman Clegg when they were all standing on the pavement in their best clothes.
As for the doubles of the trio, yes, they came in with Seymour, I think Michael Aldridge suggested the doubles could do much more than they were doing till then, aprt from the stunts. And yes, Cleggy was driving the car backwards, but was supposed to stop before it broke the fence and landed in the pond, but for some reason he didn´t and by accident drove right into it. LOL, I wish they had filmed that.