Tightening chain on Daytona 955i

It is not unknown for the adjuster thingy to seize into the s/arm. This is due to lack of lubrication in that area. I would just carefully apply a suitable hammer to the c spanner to try to break the grip, after firstly applying a goodly quantity of WD40 or similar and letting it soak in for as long as your boredom threshold/patience will allow.
Do not be tempted to apply too much lubrication to the adjuster when you tighten it up again as the pinch bolt may not be able to apply enough force to stop it moving out of adjustment. There is a fine line between to much and too little lube in that area it would seem.
Almost forgot ... the c spanner should have a sleeve extension to go over the handle in order to give a bit more leverage.

 

Almost certainly it's seized. The only real cure is a strip down and clean. This often gets overlooked especially by people who service the bikes themselves or just pay for minor services. Expect it to look like this

 

Would it be possibble to try seperating the the arm more with a large screwdriver where the pinch bolt is?
If it is done carefully, or even using a small wooden wedge, just to ensure at least one side is fully released...
Then maybe a wooden/soft mallet

 

Don't get 46mm but 1 13/16inch. cheaper and 1/2inch drive. think its for a mini stub axle.

 

Try this YouTube link

Linky

 

Cool
I hope it goes smoothly for you