Series1 and Series 2 are avaiable to buy indivdually. There are 6 volumes season 1 (13 episodes) and 9 volumes for season 2 (18 episodes).
After that they released they weren't selling that well because you were only getting 2 episodes per DVD (volume). So they made box sets for Series 3 and Series 4.
However I know they have screened on television series 5 and series 6, however neither of these series are avaiable to buy in the UK. I was asking all of you fine people on this forum if anyone knew if they are planning on releasing Series 5 on DVD.