Magnetite in combi secondary heat exchanger

Hello,
Does anyone have any advice on the best way of getting magnetite out of the secondary heat exchanger of my combi? I've just tried to descale it on the assumption that its erratic performance was being caused by limescale on the secondary side. Howerver, from what I can see it's actually the primary side that's clogged up with black magnetite flakes.
Is it possible to chemically dissolve magnetite? Or is it a case of chucking it out and getting a new heat exchanger?
Incidentally, I'm a bit baffled - the system does have corrosion inhibitor in it, and when I drained off a gallon or so of water from the draining cock to depressurise the system the water was virtually clear. The magnetite seems to be quite selective.
Cheers!
Martin
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I should add that a couple of hours with a bucket of steaming Fernox DS3 descaler, a disposable drill pump for circulating the stuff, and a lot of flushing under the kitchen tap has not made it work any better.
On the plus side, I have learnt
a) to make sure that none of the O rings have dropped off the heat exchanger before you turn the water back on (best shower I've got out of that boiler in ages) and
b) that vital screws dropped inside the boiler casing gravitate to the most inaccessible and invisible location, and that such screws are always made of some mysterious non-ferrous metal making the use of magnets entirely pointless.
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