Unfortunately, the guy who installed the overflowing (but thankfully not dangerous, yet, accoridng to the few folks who have been round) unvented system had immaculate piping skillz. But couldn't get his own install not to drip out. So it may be one indicator, but not the only thing.
Perfect, i like to see those super neat install pics as it shows they give a toss.
will give them a ring tomorrow.