Hi Norm, you are right about trying to get information on Commando Training facilities in the Seaford area of East Sussex. There are a number of oblique references to it on some sites. The three Commando units that went to Dieppe were 3 and 4 Commando (Army) and "A" Commando (RM). Most of the training appears to have taken place in Scotland at among other places Inverarry (I'm sure you have pleasant memories of that place). The Commando units along with some Canadian Soldiers of the 2nd Division arrived in the Seaford area prior to going to Dieppe.
4 Commando under Lord Lovat appears to have been the only one to have attained it's objective, the destruction of a German coastal battery. "A" Commando was descimated and was reconstituted as 40 Commando (RM) shortly after it's return.
Prior to Dieppe 50 members of the Carelton and York Regiment accompanied 4 Commando on a raid to destroy a German Search Light unit in France that turned in to a bit of a debacle when the Germans turned the Search Lights on them. So you may be luckier than you think.
"Daddy" Armstrong's son Joe is supposed to be writing his father's biography and has said that he has some interesting things to tell about his father's exploits during the war. Maybe he will touch on this episode.
I could not find any reference that the Royal Marines ran a Commando Training Centre at Seaford they state that their Commando Training Centre has always been at Lympstone in Devon and is still there.

For a nice picture of Seaford today go to this site;

http://www.seaford.co.uk/