Splat Map shader no longer working in Unity 5

Hi we have a shader for our terrain splat maps that is no longer working even though it worked fine in Unity 4?

Here is the shader and the error we're getting: Too many texture interpolators would be used for ForwardBase pass (11 out of max 10) at line 16 (on )

I know that the issue is related to it trying to pass too many textures but i dont understand why this is now a problem and how best to correct this?

Any advice would be warmly welcome :)

 Properties {
     _Splat0 ("Layer 1", 2D) = "white" {}
     _Splat1 ("Layer 2", 2D) = "white" {}
     _Splat2 ("Layer 3", 2D) = "white" {}
     _Splat3 ("Layer 4", 2D) = "white" {}
     _Control ("Control (RGBA)", 2D) = "white" {}
 }
                 
 SubShader {
     Tags {"RenderType" = "Opaque"}
     
 CGPROGRAM
 #pragma surface surf Lambert 
 #pragma exclude_renderers xbox360 ps3
 #pragma target 3.0
 
 struct Input {
     float2 uv_Control;
     float2 uv_Splat0;
     float2 uv_Splat1;
     float2 uv_Splat2;
     float2 uv_Splat3;
 };
  
 sampler2D _Control,_Splat0,_Splat1,_Splat2,_Splat3;
  
 void surf (Input IN, inout SurfaceOutput o) {
     fixed4 splat_control = tex2D (_Control, IN.uv_Control).rgba;
         
     fixed3 lay1 = tex2D (_Splat0, IN.uv_Splat0) * splat_control.r;
     fixed3 lay2 = tex2D (_Splat1, IN.uv_Splat1) * splat_control.g;
     fixed3 lay3 = tex2D (_Splat2, IN.uv_Splat2) * splat_control.b;
     fixed3 lay4 = tex2D (_Splat3, IN.uv_Splat3) * splat_control.a;
     //o.Alpha = 0;
     o.Albedo.rgb = lay1 + lay2 + lay3 + lay4;
 }
 ENDCG 
 }
 Fallback "Diffuse"
 }