Out of the six British species of Tetragnatha, two are larger and more common and one of them associates with water, T. extensa, although detailed genital examination is needed for a secure identification.
Tetragnatha have very large jaws, and the males using their modified jaws to lock the female's in place - and keep out of danger - when mating.
You can see the folded fangs in this shot
And this is a scanned figure from Bristowe's The World of Spiders (1958), drawn by Arthur Smith, demonstrating the locking mechanism (male at the bottom)
I took this shot to give you an idea of the size of the spider
For a wonderful account of the mating behaviour and natural history of these spiders, go to the Spiderbytes blog.