Let me see if I can explain this in words.

Model 15-2 95+ percent.
No pushoff...bore crisp etc...
The problem:

As you cock the revolver to check timing all is good.
With Gun cocked, trigger pressed and
holding back the hammer to check for cylinder play all
is good.

However, on de-cocking the hammer the cylinder with spin
back clockwise on one cylinder.

Thanks in advance.....

Sounds like the cylinder lock is dropping and the face of the spring-loaded hand pushing against the ratchet is camming the cylinder back.

Check that the cylinder lock is not still engaged with the trigger after lock-up; it should not be.

As the previous post has stated, interaction between the hook at the front of the trigger, and the bevel on the stop when the trigger is returning should only move the stop forward, and then release it backward. (during the trigger return stage)
You can confirm the four directions of the stop by opening the cylinder, and cycle the action while you hold the cylinder release back. During the trigger pull, the ball of the stop should drop down, pop back up. When the trigger is released, the ball should move forward in the frame slot, and then be released back to it's resting position.

After cleaning, and with a magnifier, check the cylinder stop ball for damage or excessive wear. Also, examine the stop slots in the cylinder itself, confirming they are not damaged or worn excessively, and that they are free of dirt and debris....especially at the bottom. If the stop slots are dirty, scrub them out with a nylon brush or old toothbrush. Use solvent if needed.

You should also check to see that the stop "holds" in each one of the stop slots. You can confirm this by first confirming the gun is empty.....then cock the gun SA and after cocking, put some rotational stress on the cylinder, attempting to rotate it by hand from it's locked position. The cylinder should stay locked when this nominal stress is applied.

__________________
Ret. LE, FA Instr, S&W Armorer

Many thanks......