Also, it's worth bearing in mind that in the late 1980's DC comics released a four-part story which was a sequel to
The Prisoner set 20 years after the TV series. The graphic novel collection of this,
Shattered Visage, includes a couple of pages of text that explain that the events that take place in
Fall Out as being entirely drug-induced - a further step towards the attempts to psychologically break number 6 that we see in
Once Upon a Time.
Whilst the comic story isn't a direct part of the TV series, it features the characters played by Patrick Mcgoohan and Leo McKern (with their phsical appearances) and the whole thing was supervised and vetted by McGoohan himself, so as an official sequel it carries some credibility.
Incidentally, McGoohan discusses
The Prisoner in
this interview, transcribed here from Canadian television.