I think I know the answer to this one,but...
Wikipedia says:
"Episode One" (11 Mar 1961)
"Episode Two" (18 Mar 1961)
"Pilbeam, the Journalist" (25 Mar 1961)
"Episode Four" (1 Apr 1961)
"Episode Five" (8 Apr 1961)
"Episode Six" (15 Apr 1961)
Six more episodes were made, but they were never transmitted. All twelve episodes still exist.
Lostshows says:
25.03.1961 Pilbeam The Journalist (missing)
08.04.1961 The Rhino's Night Out (missing)
15.04.1961 A Musical Evening (missing)
22.04.1961 The Sinister Neighbour (missing).
Any chance that Wiki's right?