Imagine the scene. It is August 1988 on a holiday beach near Harlech in Wales. A huge beast has washed up on the beach. It is nearly the size and shape of a Volkswagen beetle car.

In fact it is a leatherback turtle. And at just under ten foot/3m long nose to tail, and weighing in at just under a tonne it is the biggest leatherback ever seen anywhere in the world.

The leatherback sea turtle (Dermochelys coriacea), sometimes called the lute turtle, is the largest of all living turtles. 

Unlike most other sea turtles it has no bony shell. Instead, its carapace is covered by oily flesh and tough leathery skin. 

Today sightings of leatherback turtles are increasing all around the Welsh coast and in south-west England. They are are drawn here by the high numbers of jellyfish - the main diet of these huge marine reptiles.

Sadly many turtles risk death or serious injury by mistaking the many polythene bags and other plastic litter that floats in the sea as its jellyfish diet.

The leatherback turtles lay their eggs on Caribbean beaches before crossing the Atlantic Ocean to feed in Welsh coastal waters.

To those lucky enough to spot one they are one of the most spectacular species to visit our coastal waters.

On inland waters too turtles and terrapins are becoming more and more common. There is a healthy colony at the London Wetland Centre at Barnes.

I often see them in the ponds at Cambridge University botanical gardens.

The rangers on Hampstead Heath's ponds in London wage a constant battle with the sliders more at home in the waters of Florida's Everglades.

Every now and again the invasion excites tabloid journalists, "Terror Turtles" or even "Terrorpins" make very good headlines but are probably a bit exaggerated.

Even so more and more these exotic visitors - some as big as dinner plates - are being spotted on canals, slow flowing rivers and in ponds and lakes.

The main problem probably goes back to the time when thousands were bought as pets when the Mutant Ninja Turtles were the latest thing on TV and in pet shops.

The terrapins soon got too big for their tanks or nipped their young owners once too often. Just as likely the kids got bored and went on to another latest craze.

Thousands of pet turtles and terrapins got dumped in rivers, lakes and ponds. Most died but a few flourished, some even managed to breed and now we have a small but growing population of these fascinating and colourful reptiles.

Most of them are either red-eared terrapins or snapping turtles the two commonest species imported for the pet trade.

The red-eared terrapin is relatively harmless but can give a human a painful nip. Many of them also carry salmonella.

A snapping turtle is a much more aggressive animal. It will kill young ducks and a bite from its jaws can easily sever a human finger.

Britain actually has no native Chelonia, the family that includes group tortoises, terrapins and turtles except the marine turtles that visit our shores.

Any terrapins, fresh water turtles or tortoises living here are invasive species and as such can undermine established food chains and interfere with natural biodiversity.

Many years ago various desert tortoises were cheap and popular pets. They arrived from north Africa as ballast in ships docking in British ports. Most died on the journey but the few that survived were sold cheaply by pet shops.

Many a small garden had its pet tortoise and once established they could live for a very long time.

This trade in cheap tortoises forced many wild tortoise populations into extinction and now the trade has been made illegal with the animals protected by law.