Sounds like the safety valve is either indicating a problem or just leaking. Does the pressure gauge go up soon after lighting the boiler? If so the expansion vessel could have failed. I suspect you are regularly topping up the system. If the pipe leaks whilst the system is cold as well then the safety valve is leaking. If only when running them most likely the expansion vessel. (Replacement is £100 - £150 I'm afraid)
Paul Thurmaston, Leicester, May 2010