This is driving us mad. We read it in school in the 80's - paperback, landscape, beautifully illustrated. There was some sort of monster (possibly with a big red nose?) that used to cause chaos around the house and the little boy would get into trouble for it as his mum didn't believe him. There was possibly a scene where the monster steals food from the fridge, and maybe one where he is playing with leaves in the garden? We've searched and searched, and I don't think we're making it up as me and my OH went to different schools in different parts of the UK and both have distinct memories of this.
Anyone?...