yes, the Tap Stat (made by Drayton, costs about £100) is a thermostatic valve that closes the primary (I thought usually the return) under the control of a vial strapped to the cylinder at an appropriate height. It is non-electric, the vial is connected to the valve with a fine flexible pressure tube less than a metre long, so the valve goes close to the cylinder. You cannot cut or shorten the pressure tube. There is also the Cyltrol which is a thermostaic valve that goes on the primary return, and closes when the return water reaches a preset temp (also made by Drayton and service parts still available, also about £100 but probably obselescent. It is inclined to seize when old so you can't adjust it.)
the only drawback I know, is that the boiler will continue to fire up under its own thermostat, to keep itself hot, even if the cylinder is satisfied, since there is no cylinder stat as in fc's diagram to tell it to stop. this will waste a certain amount of gas.