Bayliss seem like a company that cares about its products and its customers - could be worth getting in touch to see if there's a way of checking whether your autovent is within spec (e.g. the maximum angle the operating arm should open to at 'x' degrees.)
As previously mentioned, and I can't prove it, my gut feeling is that as long as the window opens as far as in your picture, the hot air can escape almost as freely as if it was wide open.