CH cleanser and inhibitor

What is the best central heating cleanser and inhibitor? I am about to fit a couple of TRV's and will drain the system in the process. Thought, as the system is 10 years old I would flush it and buy some inhibitor for when I refill the system. I see the cost of inhibitor varies in price from around 4 up to 30. Why such a difference? What would you recommend?
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