To appreciate this series fully I think it helps to have read Reginald Hill's books. They are beautifully written, with strong plots and in parts very funny. I think Warren Clark does a pretty good job as Andy Dalziel - if anything not as 'Frankenstein' like as the written character.
If you have read the books though, the difference between the stories based on Reginald Hill's books and the others is very obvious. Consequently, series 1,2,3 and 4 plus the two episode 'Dialogues of the Dead' are the ones to collect. The later series are reasonably good TV but not in the same class.
But why haven't series 3 & 4 and 'Dialogues of the Dead come out on DVD? I asked '2 Entertain' (who released 1 & 2) about this recently but they replied that they have no current plans to issue them. They did say that they note enquiries such as mine and if they get similar enquiries they review their publishing policy. Others who are interested might, therefore, think it worth dropping them an e-mail at
DVDenquiry@2entertain.co.uk.