TV series in the UK have fewer episodes than US counterparts. A series (substitute season, for a US comparison)will be commissioned for a specific number of episodes. However, Prime Suspect was more of a miniseries, in that each episode was a component of a larger story. Most proper UK series have several different stories going on, though there may be some multi-parters and an arc over the course of the series. The first three Prime Suspects were each a single story, played out over several episodes, as in a mini-series. That changed with series 4, where there were 3 separate episodes, though with some linking aspects. Series 5 returned to a single story and the later series 6 and 7 followed suit.
Fortunately, Ah keep mah feathers numbered for just such an emergency!

thanks!

More like a group of mini-series. Two parts, roughly 100 minutes to the part. There was usually a couple of years between each mini-series.