How to turn off a Baxi Bermuda hot water on gravity fed system

[Clem61]

My parents' house is heated by a Baxi Bermuda gas back boiler with a gravity fed hot water cylinder and pumped central heating controlled by a wall thermostat. The controller allows hot water, or hot water and heating. The hot water tank is of the indirect combination variety. The gas boiler replaced a Parkray coal boiler and there is no expansion tank or vessel (I think they say it's managed by air-locks), and no cold water tank in the loft.

What do I need to do, please, in order to:
a. fit a hot water cylinder thermostat, and
b. turn off the water heating so that only the radiators operate.

THanks.