I think it's time I replenished the additives in my system since I
haven't added anything for three or four years, and have added quite a
lot of freshwater during that time, to keep the pessure correct.

What is a good and economical additive to use, and how much for a
smallish 3-bed house?

The crud that I'd most like to eliminate are those hard little black
bits (iron oxide?). I did a desludge job on it about 3 or 4 years ago,
but the black bits are back! - I found some the other day when I
looked inside the secondary heat exchanger..

Thanks

Dave L