You tried to top up the system by letting water in. The pressure got high enough to operate the system overpressure relief valve, at around 3 bar. If you are regularly topping up it is very likely that you need to recharge your expansion vessel. Whilst you could do this yourself, it is not difficult, just need a car tyre type air pump, there are no gas safety elements and you can google the instructions., you might prefer to get a gas man to do it (will cost you -may try to tell you its obsolete, cant be fixed, need a new one etc)
Watch the system pressure as the central heating goes from cold to hot - if it increases much you do need to pump the vessel up.
The overflow is the normal, safe, response to what you did (over pressurise the boiler). Dont worry.
c#
April 2011
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.