if (results != null)
{
    // Use NVC to build a Query String using internal HttpValueCollection
    NameValueCollection queryString = System.Web.HttpUtility.ParseQueryString(string.Empty);

    if (!string.IsNullOrEmpty(SearchBySolicitor.Text)) {
        queryString["firm"] = Server.UrlEncode(SearchBySolicitor.Text);
    }

    if (!string.IsNullOrEmpty(AreaOfLaw.Text)) {
        queryString["area"] = Server.UrlEncode(AreaOfLaw.Text);
    }

    if (!string.IsNullOrEmpty(Location.Text)) {
        queryString["location"] = Server.UrlEncode(Location.Text);
    }

    queryString.MyExtension("postcode", Postcode.Text);

    string URL = Sitecore.Links.LinkManager.GetItemUrl(results) + "?" + queryString.ToString();

    // Send user off to results
    Response.Redirect(URL);
}
else
{
    // Unable to find results error
    litTest.Text = "Error! Unable to find results page";
}

public static class MyExtension
{
    public void MyExtension(SomeType this queryString, string stringName, string text)
    {
        if (!string.IsNullOrEmpty(text))
        {
            queryString[stringName] = Server.UrlEncode(queryString, );
        }
    }
}