The Century 21 range of mini-albums were a staple of every Gerry Anderson fans record collection in the 60s. Each release was a 33rpm EP and featured "21 minutes of adventure". Although the 21 minute tag was correct, the term 'adventure' was more loosely applied. Some were abbreviated audio adaptations of episodes of Thunderbirds whilst others saw the listener taken on tours of Marineville and NASA rocket launches courtesy of Troy Tempest and Steve Zodiac respectively. Sometimes they were even canon-blasting original adventures for Lady Penelope and Captain Scarlet. For some reason, Century 21 stopped producing their mini-albums following Captain Scarlet & The Mysterons. There are those of us who would've appreciated the range going on throughout the later Gerry Anderson productions...
I haven't done any mini-albums for a while but I was reminded of the final four that the failing company (which had been sold off to K-Tel) released in the mid-seventies. They were based on the popular Space: 1999 and featured specially recorded performances by the three main stars - Martin Landau, Barbara Bain and the other one. Sadly the entire range was swiftly withdrawn from sale shortly after their release following complaints from parents about the John Koenig one. Can't think why...
They occasionally turn up in charity shops but not as records.