water pouring from boiler?

I have a Worcester Greenstar 24i/28i junior combi system. Water is pouring from the overflow and dripping from underneath the unit. Its snowing outside, the temperature is due to drop to about -5 within the next few hours and I have no heating or hot water. Can't get hold of a heating engineer, any idea what is wrong or what I can do until I can get someone out?
Oh, and I can't find anything to turn off the water feed to the boiler

Jax
January 2010