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?