Heat on demand boiler + tank steup?

Is there an off the shelf component that will let me sense when a ground level hot water tank is full (supplied by heat on demand boiler) and shut the supply off?

The idea is that then a booster pump will push the water to the bathroom upstairs for high pressure / flowrate.

Here's a picture of what I'm thinking of doing;

http://img27.imageshack.us/img27/2140/39052854.png

It's important to note that the hot storage tank is being filled by the boiler (not a cold tank) and that the top of it must be open to the air so the pump can suck the water out the tank rather thank trying to suck it through the boilers heat exchanger.