KurseD...
The creature you're referring to is a type of short-necked plesiosaur known as Liopleurodon, which the BBC Special reported as being 25 meters (about 80 feet) long and weighing in at a staggering 150 tons. Unfortunately for the lovers of enormous predatory beasts, the best paleontological evidence available puts Liopleurodon more in the range of 10 meters (33 feet) which is plenty big but not as enormous as the television folks would have us believe.
There is some evidence that other (possibly even larger) plesiosaur- and mosasaur-type creatures swam the Earth's ancient oceans. Kronosaurus fossils, primarily from Australia, indicate a length of 10 meters (33 feet) or more. The so-called 'Monster of Aramberri', an enormous plesiosaur whose remains were discovered in Mexico back in 1982 may have reached a length of 18 meters (59 feet). The really big mosasaurs (like Tylosaurus proriger) may have reached up to 15 meters (49 feet) or more.
Unfortunately, or fortunately depending on your perspective, the plesiosaurs and mosasaurs have been extinct since the end of the Cretaceous (about 65 million years ago), even though the legendary creatures like the Loch Ness 'Monster' are sometimes said to be relict populations of these long-gone beasties.
-Joe