@digster have you tried what Mike suggested? Tax is dependent on which country the buyer is so if you are logged in and if you are testing it using the same email as wordpress admin then it will show the correct price with tax included and if not logged in it will show the price without the tax. Setting the default customer location will fix this.
In my case I sell for AU and so I set base and default customer location to AU and it solves this.
Hope this helps someone. ??
