Just starting to play with Umbraco 8 and I am getting errors in the generated models for Umbraco 8

Error:

'File' does not contain a definition for 'Value' and no extension method 'Value' accepting a first argument of type 'File' could be found (are you missing a using directive or an assembly reference?) 

This error occurs for all models generated that use the syntax this.Value

e.g.

    ///<summary>
    // Width: in pixels
    ///</summary
[global::System.CodeDom.Compiler.GeneratedCodeAttribute("Umbraco.ModelsBuilder", "8.0.1")]
    [ImplementPropertyType("umbracoWidth")]
    public int UmbracoWidth => this.Value<int>("umbracoWidth");

I notice in Umbraco 7 the same portion of model code is

///<summary>
/// Width
///</summary>
[ImplementPropertyType("umbracoWidth")]
public string UmbracoWidth
{
    get { return this.GetPropertyValue<string>("umbracoWidth"); }
}

The models builder configuration in the web.config is:

<add key="Umbraco.ModelsBuilder.Enable" value="true" />
    <add key="Umbraco.ModelsBuilder.ModelsMode" value="AppData" />
    <add key="Umbraco.ModelsBuilder.ModelsDirectory" value="~/../Umbraco.Web.PublishedContentModels" />
    <add key="Umbraco.ModelsBuilder.AcceptUnsafeModelsDirectory" value="true" />

We follow the same pattern successfully for all our Umbraco 7 sites - we use a 3 project solution

Umbraco.Web.PublishedContentModels - with references to Umbraco.Core and Umbraco.ModelsBuilder and System.Web

xyz.Core - with reference to project Umbraco.Web.PublishedContentModels

xyz.Web - with reference to project Umbraco.Web.PublishedContentModels

Out of the box the only model that does not throw an error and allows compilation is the default folder model - as this contains no properties to return.

Any ideas anyone?