Sorry, can't help you. There aren't any hotels actually "on site" - they are all very much landside and a 20 minute bus ride away. BUT if you're flying Internationally on your connecting sector, you might think about buying access to an executive lounge.

Visit http://www.holidayextras.co.uk/airport-lounges.html and you can buy a pass for any Servisair lounge (plus a few others) at Heathrow where you will be able to wait in comfort. The official line is that your stay can't exceed three hours BUT asking the person on the welcome desk usually results in your being allowed to stay from pretty much as long as you wish.

Hope this helps a little.