When you view this page in a browser it looks fine: http://advertel.com/emails/ADSonHOLD/

But when I send the file as an HTML email there is large space below the header image and before the footer.

Any ideas?

What did you create that in, MS Word ?? It's chock full of MS proprietary garbage. No surprise it breaks. I suggest you clean that out and start fresh, the much smaller page size will be appreciated by your recipients.

Lots of good info on HTML email, including free templates here: http://www.campaignmonitor.com