Of the 430 episodes of the series, only 19 exist in their entirety in the BBC archives. The rest were wiped in the late 1960s and early 1970s. This is true of numerous other BBC series of that vintage, most notably Doctor Who and Top of the Pops.
Jack Warner (George Dixon) is the only actor to appear in all 430 episodes of the series. In second place is Peter Byrne (Andy Crawford) who appeared in 424 of the 430 episodes produced. They were also the only regular cast members to stay with the series throughout its entire run.
Warning: Dixon of Dock Green guide may contain spoilers