Its probably been months or even a year or so since the last top up so that rules out any part of the heating system as the culprit.
Just to return to the meter, it may be no harm to take a meter reading, then run off some cold water into a 10 or 20 litre drum and note the metered consumption.
Just to return to the meter, it may be no harm to take a meter reading, then run off some cold water into a 10 or 20 litre drum and note the metered consumption.