Actually, looking on the net yet again, I've found a few sites that say that TDS has little to do with GH!!
TDS embraces GH and KH as Colia originally said as well as other dissolved metals not included in GH and KH.
GH is only a measurement of all the divalent metal ions, predominantly Ca++ and Mg++.
There is a relationship between TDS and GH though especially if you are using RO water so you know exactly what is in your water!!
This whole problem came from the fact that I could not raise GH easily using Kent RO Right using a test kit. As I mentioned earlier, test kits only measure dissolved Calcium and Magnesium which normally are by far and away the most predominant divalent metals in water. However, it looks as if RO Right contains a considerable amount of other salts which do not contribute to GH. I ended up more than doubling the dosage to get a GH of 2dH!! Of course, now, my TDS value is very high, 400ppm!! I'm off to dilute this gradually!!
Every morning when I wake, I thank the Lord I'm Welsh!