I think the problem may arise when I set a flag somewhere where there isn't actually a mine. If I do this, I assume I can clear the flag by switching to "Open Field" and clicking again on the same square. This correctly opens the field, but the "Remaining Mines" count does not go back up again.

So if one never makes a mistake, there's no problem. But as soon as one has incorrectly put a flag somewhere, there's no way of undoing it and then finishing the game.

If anyone else would like to confirm or deny this theory, please just try marking a square where there obviously isn't a mine, and then opening that square - and see what happens to the Remaining Mines count.

Regards - Neil