Right.
The A100HE is a 100Amp RCD of 30mA sensitivity.
This trips when there is an earth leakage fault. It works rather like those plug-in RCD adapters people use with lawnmowers, to save them from electrocution when they cut through the cable when standing on wet grass. Yours is however better quality, built into the consumer unit and capable of carrying a 100 Amp current, and protects all your sockets. It also sounds as if it is protecting the cooker, this is not always done unless the cookercontrol has a socket on it; and it protects the water heater; water heaters are very prone to leakage when they get old and corroded. If you switch "off" the MCB for the water heater, it may reduce the number of trips, which would be a sign that it is faulty. However, I understand you to say that the trip occurs when you turn on the boiler.
This is a guess because I am not in your house looking at it; but since it appears that the RCD trips when you turn your boiler on, it is quite likely that there is an electrical fault in the boiler or its controls. It might be due to a water leak reaching the electrical parts, don't know. Best thing to do first is to get someone in who is skilled in repairing that type of boiler, he may well find and fix the fault quickly. If not, the next step would be to call in a qualified electrician. One of the things that sometimes happens is that you get several small earth leakages, no one appliance is very bad on its own, but they add up and together will trip the RCD. These faults are more difficult to trace, and need professional testgear to find and fix.
If you know of recent water leaks; or damage to cables or controls (including drilling or nailing into walls); or mice getting into the kitchen and gnawing at cables, that would be a clue to a possible cause.
