The system tries to fire up for a couple of seconds then switches off. If you press the "overheat thermostat reset button" the system fires up OK. The next few times the system will fire up on its own, then the fault repeats itself. The boiler is 16 years old and has been serviced annually.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.