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That worked for me when I had to replace the siphon in my toilet (the diaphragm was ripped so even a manual pull of the metal bit didn't make it flush properly).
Only tricky thing, from memory, was the need to undo a massive plastic nut (60mm or so in diameter I think) to be able to remove the siphon. I ended up cutting a bit of wood into a makeshift spanner as nothing I had would fit something so big.
Greenbank-
Usually pump pliers work on those nuts, just to free them then they should turn off by (a gloved) hand.
When you are replacing a flush mechanism it's a good idea to hoover out the water from the bottom of the cistern as the flush never gets rid of it all.
Check if the handle is pulling the arm up far enough, you can shorten the flush mech arm by bending the top a bit more.
Loving the bit of swimming float that's saved someone from having to replace the ball cock. Probably couldn't find a ball cock that would fit in the cistern on that arm and couldn't face the dodgy inlet valve plumbing. I'm seriously remembering that bodge for the day I might need it.
Airhead
If you need to empty the cistern, just stop the ball cock from dropping and the valve won't open. Flush the water out, job's a good'un. I don't see why you'd need to stop the supply anywhere else.