There is a great explanation here that I hadn't considered: http://answers.unity3d.com/questions/122586/layermask-vs-1ltlt-gameobjectlayer-.html

I need to 'cast' the object's layer, which is the integer shown in the inspector, in to the masked version, which is used to compare with the LayerMask. For example, layer "6" in 1, 2, 3, 4, 5, 6** needs to be 1, 2, 4, 8, 16, 32**

Here is a working function to do this:

/// <summary>
/// Checks if a GameObject is in a LayerMask
/// </summary>
/// <param name="obj">GameObject to test</param>
/// <param name="layerMask">LayerMask with all the layers to test against</param>
/// <returns>True if in any of the layers in the LayerMask</returns>
private bool IsInLayerMask(GameObject obj, LayerMask layerMask)
{
    // Convert the object's layer to a bitfield for comparison
    int objLayerMask = (1 << obj.layer);
    if ((layerMask.value & objLayerMask) > 0)  // Extra round brackets required!
        return true;
    else
        return false;
}