sorry, I was thinking of the tap supply, one Christmas we had to pull all the kithen units out and all the copper pipe behind them, another year it was the cylinder, being Christmas/new year of course we couldn't get the parts, spent about 2 weeks wriggling round units in the kitchen (OH is heating engineer, he was too busy mending other peoples heating
)
Ours is only the Royale, heats the S/S cylinder immediatly above upstairs rather than straight to the taps, so that's a sealed system with inhibitor in.
The central heating is seperate and as you say, inhibitor should work.
We also installed a year or so ago, a filter system, with UV? light, supposed to sort any bacteria and deals with the Ph. really wierd going into the garage in the dark, this strange green glow in the corner.