The UK is home to four resident Darters (Sympetrum).
Two, the Common Darter (Sympetrum striolatum) and Ruddy Darter (Sympetrum sanguineum), are widespread and somewhat similar, so are a frequent source of confusion.
The remaining two are more restricted in their distribution. Of these, males of the Red-veined Darter (Sympetrum fonscolombii) are similar enough to the first two to increase the confusion. Males of the fourth, the Black Darter (Sympterum danae), are, as the name implies, black and thus distinctive.
Females and immature males are all essentially yellow and can readily be confused.
To keep my id charts down to a reasonable size, I have split the males and females.