Up until now my instance of Umbraco only housed one website so to get the root node I would simply use the code:
var context = _umbracoContextAccessor.GetRequiredUmbracoContext();
var site = context.Content.GetAtRoot().First();
But now I will be adding adtional sites to the instance which will be using the same doc types and templates as the existing site. So how can I get the correct root node for the site the user is visiting?