Hi. I am completely refitting my bathroom in my 1980s home. I would like to remove the soil pipe that goes up behind the toilet, through the ceiling and out of the roof. This would create more space. Can i build a box behind the toilet as before but only as high as to support the cistern and then fit an AAV to the soil pipe inside the box? I would make sure the top of the box was removable for easy access. There is a waste pipe as shown on the picture from bath and basin then similar one downstairs from the kitchen sink. There is a Floplast AAV in Screwfix which i think will do the job. Thanks.