What a coincidence with the wheels. The OSF wheel is always lower than the rest on mine, same st wheels as yours on my standard 1.6 focus. So I found that odd, and queried it with a tyre fitter. He said it could be a buckle in the wheel allowing the air to escape. It's fine once it's kept topped up. I don't believe it's the valve or tyre as they have previously been looked at, by a few fitters.