Sorry, you've lost me completely!
In the circumstances you describe, the water in the cylinder is *already* hotter than the cyl stat setting, and the the cyl stat has already turned off - but the boiler and pump continue to run for the benefit of the CH. What further effect can the cyl stat *possibly* then have when the room stat turns off?
Please explain what you mean by this 'slug of water'. Are you referring to the water in the boiler continuing to get hot after the boiler has stopped firing, due to the residual heat in the boiler's heat exchanger? If so, this is hardly going to cause the water in the hot cylinder to boil.