Two tensioners rebuilt and ready to go back in.
The hardest part was getting the lower circlip back in. In the end I used a bicycle tyre lever which was the perfect shape to hold in one side of the circlip, keep the plunger down while I worked the other side of the clip with a small screwdriver. The first one took me at least an hour of trying. The second one took 30 seconds as I knew my approach.
The other trick was using line trimmer cord to push down the ball valve to bleed the air, and using a coin to press the end of the plunger to save the thumb.
As others have posted, it's pretty straightforward but having the exploded view in the Haynes manual really helps when you are reassembling to make sure everything goes back in order.
The tensioners are rock hard now and cannot be moved by hand. Now comes the part where they are compressed ready for fitting. I will use the vice or a clamp for that, and I have the special tool on the way.
Edit : I got some gasket cleaner today. That stuff is toxic but works. I put a small nick in the metal on the outside edge while scraping, but I think I'll be ok.