Thanks for all your replies. This is what I did in the end. Got out the ""Instruction Manual"" and re-read it, after it being dormant for some while !! It is all self-cleaning, with a setting to use to reclean the stay-clean sides, back and ROOF. I must admit I have never used this setting.
I got a soft large toothbrush (I keep a selection of them in my cleaning box) and just gently rubbed around the grill bars. Nothing came off onto the brush or the papers I had placed in the bottom. So I now feel pleased to get all your hints, and that I have successfully done it. Thanks again, Mrs Bouquetx