Hi Chris
Did you compress the pin the in tensioner before fitting it on the car? You need to compress the pin into the tensioner (with a vice or similar) and then slide a small allen key or somesuch (drill bit?) through the holes in the body of the tensioner and the hole in the pin so that the pin is held back until you have refitted the tensioner. Once it's back on the car you can pull the allen key out, hand grenade stylee, and it will adjust the cambelt tension correctly. If you don't do it this way the cambelt tension will be way to high and may result in a snapped or prematurely worn belt.
Apologies if you already knew this. Better safe than sorry