Not sure what might be available on your side of the pond, but I typically use Prestone super flush (something like this: Prestone® Radiator Flush + Cleaner - Prestone® Total Protection). Pour it in and run the engine with the heater on and drive it for a couple of hours if possible, then drain and let the engine cool down completely. When cold, refill with distilled water and let it run until hot, then drain again and let it get cold. Check to see if the drain water is now clear and not rusty looking. If it looks good, then fill with distilled water again mixed with the appropriate antifreeze product.