UI with no Ztest appearing behind geometry when masked.

So I have a world space canvas for a HUD which has a load of UI with a no ztest shader. All of the UI works and appears in front of all geometry unless I mask it.

I did have to follow the following to get the mask to work (clearMask gameobject in the screenshots below), as im in deferred rendering: http://forum.unity3d.com/threads/ugui-masking-blocked-by-background-meshes.270122/

Here's my hierarchy:

Here in the bottom left corner you can see the masked element being drawn behind geometry:

Here's the UI shader im using (applied to the HUDUI material): Shader "UI/Default_OverlayNoZTest" { Properties { _Color ("Main Color", Color) = (1,1,1,1) _MainTex ("Alpha (A)", 2D) = "white" {}

         _StencilComp ("Stencil Comparison", Float) = 8
         _Stencil ("Stencil ID", Float) = 0
         _StencilOp ("Stencil Operation", Float) = 0
         _StencilWriteMask ("Stencil Write Mask", Float) = 255
         _StencilReadMask ("Stencil Read Mask", Float) = 255
 
         _ColorMask ("Color Mask", Float) = 15
         _Illum ("Illumin (A)", 2D) = "white" {}
     }
 
     SubShader
     {
         LOD 200
 
         Tags
         {
             "Queue" = "Transparent"
             "IgnoreProjector" = "True"
             "RenderType" = "Transparent"
             "PreviewType"="Plane"
             "LightMode" = "Vertex"
         }
 
         Stencil
         {
             Ref [_Stencil]
             Comp [_StencilComp]
             Pass [_StencilOp] 
             ReadMask [_StencilReadMask]
             WriteMask [_StencilWriteMask]
         }
 
         Cull Off
         Lighting On
         ZWrite Off
         ZTest Off
         Offset -1, -1
         Blend SrcAlpha OneMinusSrcAlpha
         ColorMask [_ColorMask]
 
         Pass
         {
             CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
                 #include "UnityCG.cginc"
 
                 struct appdata_t
                 {
                     float4 vertex : POSITION;
                     half4 color : COLOR;
                     float2 texcoord : TEXCOORD0;
                 };
 
                 struct v2f
                 {
                     float4 vertex : POSITION;
                     half4 color : COLOR;
                     float2 texcoord : TEXCOORD0;
                     float2 uvIllum : TEXCOORD1;
                 };
 
                 sampler2D _MainTex;
                 float4 _MainTex_ST;
                 fixed4 _Color;
                 float4 _Illum_ST;
 
                 v2f vert (appdata_t v)
                 {
                     v2f o;
                     o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                     o.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
                     o.color = v.color;
                     o.uvIllum = TRANSFORM_TEX(v.texcoord, _Illum);
 #ifdef UNITY_HALF_TEXEL_OFFSET
                     o.vertex.xy += (_ScreenParams.zw-1.0)*float2(-1,1);
 #endif
                     return o;
                 }
 
                 half4 frag (v2f i) : COLOR
                 {
                     half4 col = i.color;
                     col.a *= tex2D(_MainTex, i.texcoord).a;
                     col = col * _Color;
                     clip (col.a - 0.01);
                     return col;
                 }
             ENDCG
         }
     }
 }