To get a replacement disc you need to send your current one to:
33 Foley Street
The just released K9 Tales DVD set has a problem at the end of Episode 3 of The Invisible Enemy that causes scenes to play out of order.
the fault kicks in at around 21' 10, when Tom says "Get out of my head!" - there is a leap to the final lab scene where the "shrimp" appears. this continues to 22' 07 and the beginning of the "sting" when we return to the scene in Tom's head, until 22'27, when the credits kick in suddenly. this is all too noticeable and makes the end of the episode total nonsense.