Pearl,
If it is the boxed sets then I think you are right . I had a mail from Amazon today, as I have bought other Box Sets from them before . On the mail they have links to the pages for the DVD's, strangely one is described as series 29 and 30 , the other is 31 and 32 . I am not going to go into the old discussion about the episode numbering but for 29/30 the narrative on Amazon's site lists the episodes as starting with Enter the Finger and ending with Goodnight Sweet Ferret which does equate to 29/30, they have the release date as May 16th.
The description for 31/32 in blank but for mention of including the exclusive pilot episode . Assuming the list for 29/30 is correct then the final DVD can only be the last 6 episodes, unless they have found some unaired episodes but I suspect not . As they have mentioned the pilot I believe your deduction is correct . The release date they have for effectively the six series 31 episodes and the pilot is July 11th . Concerning aspect is they have 29/30 priced at £17.99 but for 31+pilot at £26.99 which is expensive for a single series , albeit the last ever.