For now I'm just selecting the last cookie value in the collection, although I realise this is a massive hack. Any better suggestions very welcome.
string value = "";
for (int i = 0; i < HttpContext.Current.Request.Cookies.Count; i++)
{
if (HttpContext.Current.Request.Cookies[i].Name == "UserCookie")
value = HttpContext.Current.Request.Cookies[i].Value;
}