I have a 2019 Octavia (Production date 10/12/2018 from VIN decode) but I have the "Mit Silikat" G13 coolant resevoir.What we do is add a flush, run the engine and drop the coolant into a tray and then a drum for recycling. We then take off a few water connections and pressure wash the system in both directions and finally blow through with compressed air. Then we change the matrix and refill with G12 Evo, we don’t clean them as it doesn’t work quickly and we have to judge the cost/benefit against a new part with a two year warranty. In the coolant tank you may find a silica bag or tube, these has been blamed for causing the gritty powder which blocks the matrix but not in the way people think. The G13 coolant suffered premature “silica fallout” and those bags boosted the content but they expire after about 5 years. Also G13 really objects to the use of tap water so G12 Evo comes ready mixed, you need around eight bottles and it is about £6 a bottle. We have started to notice a pattern in the blocking of the matrix, it happens at about 7 years so 2 years after the first cam belt change where the coolant invariably is diluted with tap water and that is a practice now banned in our workshops, everything gets G12 Evo which is the factory fill post 2017; it will be interesting in a year or so to see what it’s introduction brings.