How to reference downloaded assets within a WebView in a shared PCL Project

I am downloading a zip file from our server which contains html files, images, css etc. unzipping the files and saving them within the PCL using PCLStorage nuget package.
To view the content I am using a WebView to display the html that has been loaded successfully via reading in a stream and converting to byteArray to get at the HTML.
The problem is that relative references to images, css & js do not work even when setting the baseUrl for the HtmlSource using a dependency service as suggested in other support topics. (tested on Android and iOS simulators)
How can I load these downloaded assets in the WebView?