Moving my code behind to a class file

I have a few pages that build an identical menu to show on the page.
It occured to me that my code behind pages were repating the same code and that I should move the block into my class file and call it from there.
It works by gathering some data and then putting it in the InnerHTML of some named <span> areas.
The data returned and the span IDs are identical acroos all pages.

My problem is that once the code is moved from behind the pages spanName.InnerHTML is no longer recognised.
How can I say that the span I'm referring to is in the page that called this method?

Part 2: This menu is static 95% of the time. Is there a way I could cache this data and then have the menu built from this on subsequent pages rather than keep going back to the server (data comes from sharepoint via web services).

Code in code window

/* This is the code that I moved from the .aspx.cs to the .cs file */

public string BuildMenu(object sender, System.EventArgs e)
{
string MenuTitles = "";
StringBuilder sb = new StringBuilder(MenuTitles);

string theTopics = "";
theTopics = MakeMenu("Topics", CAML query removed for readability);
sb.Append(theTopics);
TopicMenu.InnerHtml = sb.ToString();
sb.Clear();

string theIssues = "";
theIssues = MakeMenu("Issues",  CAML query removed for readability);
sb.Append(theIssues);
IssueMenu.InnerHtml = sb.ToString();
sb.Clear();

string theObjs = "";
theObjs = MakeMenu("Objectives",  CAML query removed for readability);
sb.Append(theObjs);
ObjectivesMenu.InnerHtml = sb.ToString();
sb.Clear();

string theZones = "";
theZones = MakeMenu("Zones",  CAML query removed for readability);
sb.Append(theZones);
ZonesMenu.InnerHtml = sb.ToString();
sb.Clear();
sb = null;
}


/* This is how the code is called from within the aspx.cs file on page load */

ExtractListData etl = new ExtractListData();
etl.BuildMenu();

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