Theres a selection of those living in the lakes on the Cambridge Science Park. Rumour has it they were dumped by kids from the nearby estate during the 'Teenage Mutant Ninja Turtle' craze once the novelty of them as pets wore off.
I'm told they are hardy enough to survice but unlikely to breed successfully so although steadily getting bigger will eventually die off.
There are quite a few out sunning themselves in BCN canals in good weather.