You're right gurglypipe!   Amongst other things I use OSM data in Mapsforge format to render on the Oruxmaps app.  It appeared to me that the river was being rendered above the railway viaduct in Wetheral so just wanted to test if the absence of an explicit layer 0 was the issue.  But on closer examination it is not the issue I thought it was.  I could change this back but I don't know if it is worth creating another change set to remove something which is admittedly surplus but not incorrect.  Happy for you to remove it if you wish.