These familiar animals have been popular in the pet trade and, in the past, were imported in huge numbers. The trade has tended to focus on the attractive hatchlings. Sadly these attractive hatchlings grow into less attractive and harder to maintain adults. These unwanted adults have often been 'dumped' into local ponds.
Many terrapins originating from the pet trade are capable of surviving the UK climate, but they do not seem to be able to breed successfully. Hence, sightings tend to stem from released individuals. Occasionally, especially in publicly accessible ponds in residential areas, they are found in some numbers, which seems to be a result of many pets being dumped in the same pond - possibly for 'company'.
There is no current estimate of numbers in the wild, but it seems likely that thousands have been released across the UK.
Many released terrapins are red-eared terrapins Trachemys scripta elegans (shown above) from North America. The import of this species into the EC is now illegal but the animals are long-lived and can still be found in the UK, as can other terrapin species.