Perhaps, in the form of No.6, the Village did really catch a spy or agent, whether John Drake or not. In A B and C we observe that the Prisoner-No.6 had associated with spies in his fomer life. Having escaped back to London in Many Happy Returns the Prisoner-No.6 gives his name to Mrs Butterworth as Peter Smith. Hammer Into Anvil No.6 uses two code names, D6 for himself, and XO4 to whom is supposed to be his superior. In Do Not Forsake Me Oh My Darling we discover that the Prisoner-No.6 has even more personas used in different countries around the world Schmidt, Duval, and what's more he has a code name ZM73.
The Prisoner-No.6 is also used to working with codes and cyphers, as he does in Hammer Into Anvil, and in Do Not Forsake Me Oh My Darling it is only the Prisoner-No.6 who can crack the Seltzman code - the key relating to a series of slides which corresponds to the lettes of Seltzman's name.
If the Prisoner-No.6 was a spy, either abducted to the Village on purpose, possibly as a plant, then in Many Happy Returns he might be playing a double game, returning to London to check out that end of the operation. But that cannot be the case surely, because if it is, he certainly puts himself through a great ordeal to acheive this. Mind you, if No.6 was actually working for the Village, but had to maintain his cover in the Village as a prisoner, then his esape would have to look the real thing, and that would account for the boat M.S Polotska keeping pace with the Prisoner on his raft as a support boat!
"Number Six - a plant?!"