Where is the isolation valve?

I have a ferroli optima 700. I recently had a leak which stopped my boiler from working completely (no hot water or central heating). The engineers came out and said that my thermocoupling had gone, and I had a leaky auto air vent. Both of these have been changed, and I have now been told that the leak is in the isolation value. Im losing pressure all the time, can you help?. Also can you fill the boiler while its on? And what's a good boiler to buy?

Melinda
February 2009