You just need to continue some XPATH extraction work from where you are. There are many possibilities. The difficulty is all the yfnc_tabledata1 nodes are at the same level. Here is how you can do it (in a console app example it will dump the list of symbols and companies):

HtmlAgilityPack.HtmlDocument page = new HtmlWeb().Load("http://uk.finance.yahoo.com/q/cp?s=%5EFTSE");

// get directly the symbols under the 1st TD element. Recursively search for an A element that has an HREF attribute under this TD.
var symbols = page.DocumentNode.SelectNodes("//td[@class='yfnc_tabledata1']//a[@href]");

foreach (var symbol in symbols)
{
    // from the current A element, go up two level and get the next TD element.
    var company = symbol.SelectSingleNode("../../following-sibling::td").InnerText.Trim();
    Console.WriteLine(symbol.InnerText + ": " + company);
}

More on XPATH axes here: XPATH Axes