I did a 206 a few years ago, and I did it the same way I always do cambelt changes. Forget timing marks. I simply put a white mark on each sprocket and corresponding marks on the belt. Then remove the belt, lay it on top of the new belt, transpose all the marks, and fit the new belt aligning with the marks on the sprockets.
I can't recall the tightening procedure, but I certainly didn't use any special tools.
PS the only difficult part of the job was getting the plastic inner wing off to get access. The "captive" nuts were rusted up but turned inside the plastic housings. I think I ended up hacking it free and then doing some kind of bodge afterwards.
The special tool for setting the tension is not needed: I changed Peugoet 106 1.1 petrol using a internal house door square rod about 10cms long: fits perfectly in square hole in cambelt tensioner. Fitted 1.5kg weight on 80mm from the end another rod connected at right angles to the square one. (as described in Peugeot 106 manual)
Peugeot dealers use a special tool to tension the timing belt. A similar tool may be fabricated using a suitable square-section bar attached to an arm made from a metal strip; a hole should be drilled in the strip at a distance of 80 mm from the centre of the squaresection bar. Fit the tool to the hole in the tensioner pulley, keeping the tool arm as close to the horizontal as possible, and hang a 1.5 kg (3.3 lb) weight from the hole in the tool (see illustration). In the absence of an object of the specified weight, a spring balance can be used to exert the required force, ensuring that the spring balance is held at 90° to the tool arm. Slacken the pulley retaining nut, allowing the weight or force exerted (as applicable) to push the tensioner pulley against the belt, then retighten the pulley nut.
I thought it might be something simple as you discribed.
But can anyone answer, 'What is the other tool as discribed in the Haynes Manual used on the Rockers', what does it do and is something similar needed to replace the Belt. The manual don't explain its purpose.
I can understand the electronic measuring device is for setting the belt tension to the book, but am concerned what the other tool as discribed above is for.