I bought a zanussi electric oven with catalytic so called “self clean” oven liners a year ago and am pleased with it EXCEPT for the catalytic liners!. The bottom of the oven is smooth enamel and is very easy to clean with soap and warm water. However as much as I try to eliminate the black residue/dust on the catalytic liners the cloth always remains black…it is driving me mad!. Enamel as per the bottom would be much easier to clean. I did as, per the manual,l clean and “burn in ” the oven for an hour before first use. Is this black residue normal and will always be there?. and what about if it get on the food?,. Each time I slide a tray out some dust falls from the sides onto the oven base. CAN I vacuum the sides to get rid of the dust?…the sides are a very rough service so it is very difficult to wipe a cloth over them!….advice appreciated….thanks