Hello Rodney,
As far as I know ( and I might be wrong) there is no current importer for Atwood Products into the UK - They are an American company.
You must use the correct flue terminal, as the combustion relies on the terminals characteristics to help draw the products of combustion away from the heat exchanger.
If you are considering trying to use a none Atwood approved terminal, there may be some difficulty I obtaining a good seal to the flue because Atwood will be using imperial specifications, where as Truma for example are metric and the differences in sizes may lead to a leak through the room seal.
As Fleetwood were the origional installers, they should have some contacts for spares, but failing that, a quick search on the internet using "Atwood heater*" as the search string has brought up a number of Atwood related sites including:
http://www.atwoodmobile.com/
You may be able to obtain the parts direct from them in USA.