Found an answer, for anyone else looking:
float3 lightDir = normalize(_WorldSpaceLightPos0.xyz);
The final code looks like this (Like I say I'm really new to shader programming so this may not be the 'right' way to do things but it does work)
In CustomIncludes.cginc
float4 applySpecular(in float3 n, in float4 col, float3 lightDir, float4 lightCol, float3 ray, float specularHighlight, float specularIntensity) {
float3 reflected = ray.xyz - 2.0 * dot(ray.xyz, n) * n;
float specular = max(dot(reflected, lightDir), 0.0);
specular = pow(specular, specularHighlight);
col += specular * lightCol * specularIntensity;
return col;
}
in Assets/snippets/materials/specular.asset
float3 lightDir = normalize(_WorldSpaceLightPos0.xyz); $color = applySpecular(normal, $color, lightDir, $lightCol, rayDir, $specularHighlight, $specularIntensity);
return $color;