hi there... any experts out there? i'm about to install a new coram shower tray but just wanted to check how to correctly install the shower trap. we've had problems with previous installation so thought it best to do it ourselves! it's a top access mushroom flange trap with one rubber washer. Does the rubber washer sit on the top side of the tray or the bottom... and do i need to use any silicon sealant? if so, which side? (the top, underside or both) Any advice much appreciated!!