OK, I've changed the Model so that DomainPages and ContentPages are IEnumerable and when I run in debug mode and check the controller they are getting populated correctly.

I've got @model IEnumerable at the start of my view but I'm still getting an error
The model item passed into the dictionary is of type 'admTest.Models.SitePageViewModel', but this dictionary requires a model item of type 'System.Collections.Generic.IEnumerable`1[admTest.Models.SitePageViewModel]'.

Model

 public class SitePageViewModel
    {
        public IEnumerable<SitePage> DomainPages { get; set; }
        public IEnumerable<SitePage> ContentPages { get; set; }
    }

Controller

 public ViewResult Index()
        {
            var Model = new SitePageViewModel();

            Model.DomainPages = (from d in db.SitePages where d.SiteId == null select d).ToList();

            Model.ContentPages = (from c in db.SitePages where c.SiteId != null select c).ToList();
            
            return View(Model);
        }

View

@model IEnumerable<admTest.Models.SitePageViewModel>