The short answer
Watch Series 1 episodes 1–6, then Series 2 episodes 1–6, then Series 3 episodes 1–6. Each episode has a self-contained legal problem, but Simon’s disappearance, Honor and Daniel, and Peter’s family history develop in sequence.
Series 1: the mystery begins
Start with the premiere dated 22 April 2007 and continue through the finale dated 27 May. The final episode leads directly into the changed household found at the beginning of Series 2.
Series 2 and Series 3
Series 2 ran from 13 January to 17 February 2008. Series 3 followed from 7 June to 12 July 2009. Do not skip either finale: both contain major developments in the continuing Kingdom family story.
Where the behind-the-scenes special fits
The ITV3 making-of programme was transmitted after the Series 1 finale on 27 May 2007 and appears on the first-series DVD. Watch it after Series 1 if you want to preserve the original sequence.