clean system

Whats best to use to get rid of any flux etc in the pipework? System volume is about 400 litres (thermal store) so I'm reluctant to chuck 4 litres of Sentinel X300 (£17/ ltr) through if 4 litres of Toolstation own-brand (£7/litre) will do the job. TIA

Regarding: Clean sealed central heating system, powerflushed and inhibitor added a few years ago and system water still remains clear as tap water. Question: After cleanly installing a few brand new radiators with no flux, in reality is cleaner really required before fully flushing the system...

So I am having one of my rooms tanked and re-plastered as it has had severe damp issues. Well about two weeks ago I fitted a SpiroTech MB3 Magnetic filter to my boiler (Vaillant EcoTec Plus 937) along with chemically cleaning the system followed by draining, refilling and then adding inhibitor...