While we're on the topic ... I have a Neff oven with supposedly self-cleaning liners on the side, on which you are not meant to use anything but detergent. It has never been cleaned since we bought it 4 years ago and I am thinking it is probably about time. How do I do it? The stuff burnt onto the bottom looks impenetrable. The grill at the top seems to give off lots of smoke when used, but how do you clean that bit? The manual has a cryptic comment along the lines that you should not use any of the products designed for the purpose when the oven is warm - God knows what that means...