I believe it is Rotherfield Park, East Tisted, Hampshire, England.
I found it on the "http://www.dicamillocompanion.com". The site is the Di Camillo Database of Country Houses. If you do a search for a house (via "The Database of Houses" and "Search the Database") and put Poirot in the "Movies" field it comes up with seven country houses that have hosted a Poirot episode. There is lots of other information about each of the locations. This new information should keep us entertained for a while.
Sadly, Marsdon Manor remains a mystery!
P.S. Anybody have any idea of the identity of the Art Deco house in the Underdog.....