The left-hand sliding door of my RHD Devon ('71) has dropped about a cm down at the rear end. It takes a small heave to slide it shut and when it shuts there is a small gap, which of course allows rain in. I'm thinking the rear runner has worn out. Does anyone know if the rear runner is on a wheel or is it a nylon guide, and are they prone to dropping when knackered (like me). Thanks all. L