Thanks for the info, mate.
Is that Silver solder that you are talking of, like the Silphos that we use here to weld Copper Refrigeration pipes?.
There was a massive failure afew years ago with a crimp-fitted plastic tubing system; there is a new version out now, but I'm still very skeptical.
I would be too John, you wouldn't believe how many houses over here, had thier ceilings collapse due to what was known as a Compression coupling system for PE piping, ultimately made for the DIY plumber, but you'd expect better quality in fittings than that!.
Over here, the traditional Cold Water Cistern in the roof is slowly being replaced by the Ajax (Pressure-Reducing) Valve.
These are OK, as long as they are installed correctly and have a relatively stone-free supply.
But, If I could count all the times that I have been sent to a new house because there is no Hot Water and to find the Ajax valve blowing water (through the H/W overflow) onto the roof!.
BTW Paul, back siphonage is prevented here by having the overflow on the toilet cistern lower than the bottom of the ballcock valve and a tube leading down from the outlet of the ballcock valve.