I have the same problem, I had it tested with what sounds like the same instrument as you had, it is a plastic tube with a squeezy pump at the top and 2 cylinders both with blue fluid in?
I think the garage are having you on, because it is only a sign of HG failure when the bottom cylinder turns yellow (not light blue).
My coolant tank is 1/3rd full aswell, and the warning comes on everynow and then, i have been topping it up and again it goes down, but now i've left it, and the level does not go below the mark its at (1/3rd), i think it just settles at this level, dosent like anything more than that, i've just left it how it is now, and keeping an eye on the engine temperature to make sure it dosent get too hot. Its been 3 days and so far so good, no overheating, and no more loss (never goes lower than about a 3rd) so im touching wood!