I don't normally bother cleaning plate hexs, just replace, but recently when my father's Vaillant needed attention I cleaned it with excellent results. The hw performance was dire, even with hw dial on boiler on max and taps running slowly.
When I removed the hex, there were rust flakes in it and undoubtably the hw side would be scaled as well due to hard water. I attached a hose to each connection in turn and flushed.
I bought a small packet of citric acid from the chemist, and mixed it with about 1 pt of boiling water and poured into both sides of hex and left overnight. Poured away and flushed again.
Next, I mixed about 1/2 cup of water with 3/4 pt of Spirit Of Salts (from ironmongers) and poured into both sides of hex. Left for just over 10 mins. Carefully tipped out and flushed hex again.
Put it back in boiler, and it was good as new. Just make sure you wear goggles and mask with acid, especially the S O S. Mix acids in a Pyrex jug.
Some people have used sonic cleaners that you can buy (Ebay), but I haven't any experience of this or how effective it is.