I ha spent some time Googling these things and it seems they are pretty much a con, without any reproducible scientific evidence to prove they work. Someone I asked about them pointed me in the direction of a similar con trick from the past. The spark plug enhancer was a popular car tune-up scam. It consisted of two screws embedded in plastic with the threads exposed. You were supposed to cut the HT lead and insert the device - it allegedly resulted in massively increased MPG and was an absolute snip at just £14.99 So, no inline scale reducer for me then even at a £50 supplied and fitted.