Central heating system / boiler does not shut off
Here in the USA. we now only use 120volt control systems since it has been found that 230 volt control systems can cause problems similar to what you are describing. Many years ago we utilized 240 volt controls since it was easier to use 2 legs of 240 volt 3 phase systems to get control power. After it was discovered that these systems caused numerous drastic system failures , they were outlawed in favor of 120 volt control systems. I witnessed many failures; (overheatings, explosions, meltdowns, etc.). The wiring print you show is pretty simple so I have to ask "is this the whole wiring system showing all the controls or just a small portion of the whole system". One part of the problem of 240 volt controls is that if a short to ground develops after a gas valve opens then the valve may be held open if only one leg of the power is switched off. The valve could be held open indefinitely. There are other possible problems with your system, this is just 1 of them. I would also recommend that all the wiring be checked by a master electrician with extensive control training/service to make sure that everything is wired correctly, especially if the controls have been replaced or serviced recently. Trying to service your system from Pittsburgh, Pa. USA. is impossible but I can only render some possibilities.