sounds like a problem with the expansion vessel, whats the system pressure like when cold then again when hot?
make sure the expansion vessel is charged, and not leaking from the valve (if it is leaking at valve, it will have to be replaced)
reasons for water through safety valve would be the water pressure has exceeded 3bar, make sure system pressure when cold is 1- 1.5 bar, if thats ok, monitior it when heating is on, if it then exceeds about 2.5 bar, it points to the expansion vessel either faulty or undercharged.
Also, if system pressure is going up and down when cold over a period of time, your filling loop may be passing water (hence your system pressure wil tend to go up when mains water pressure does (higher at nighttime when water demand is low)
let me know how you get on
T
June 2008