Perhaps The Prince Should Sort Them Out.
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For more on marking an answer as the "Best Answer", please visit our FAQ.The pressure drops because of weepage which normally evaporates or more serious leaks which are more obvious. This can vary but if you are having to top up more often then say every two or three months then there is probably a leak or other problem.
This can be in the boiler or the heating system. If it is not obvious then check around all the system and boiler when it is cold for wet spots or lime scale build up. Other causes are lack of air pressure or failed diaphram in the expansion vessel preventing normal expansion and leaking safety valves.