Your post is of interest and i will hopefully reduce the fear factor. In about 1975 i was involved in the building and P & H installations on 11 properties, one of which i dwelled. The P & H was a standard two pipe system with 13 rads and gravity flow and return to cylinder, both feed by cisterns in loft. The boiler being an 80.00 BTU idea gas, no additive were used. I when back to the road just a few months ago to reminisce, and meet a pal that still live in one of the houses. We had built them together, during tea and biscuit we jawed. It appears the P & H system had not been altered, all the same as when fitted (other than too rads that were changed for cosmetic reasons) and no problem encountered. Now i'am and was standard turn it into cash plumber. With the MEC trades its been notice there money to be earn't and many scientists and component manufactures have joined the grave train.