In toto, five series were made. The first three and a half series were good; they had freshness with characters you could identify with, who worked as a team. Their boss was a real leader of independent mind, who supported his team to get the job done, and was not a management lackey. This City and South team worked closely with another team at Heathrow Airport, resulting in interesting plots and interplay of characters.
Each series features an extended intriguing detective story from the world of customs and excise, filmed in the era before mobile phones and the internet. Regular police also appear, but they are not the main focus.
In series 4, the team leader was changed to an ambitious management appointee to be replaced in series 5 in a reshuffle with a new team going by the name of Indigo and an even more ambitious leader. The charm has gone.
In series 5, each episode was now an individual story. The team cohesion has gone and the boss is abrasive and sarcastic. The nature of the story had changed with a move to a more James Bond type of action and explosions. Familiar tropes now appear – mole in the team, chasing along rooftops, female team member kidnapped by bad guys etc.
I recommend the early series for the solid plots and acting. They will also bring back memories of that time period and, if you like planes, there any many shots around Heathrow Airport.