Show them the relavent regulations, and try and explain what bonding does, the trouble is customers these days know little or nothing about the dangers of poor installation practices, now GAS thats a different ball game, there's Gassafe and thats it none of these cash cowboy scams to worry about, if there is no bonding in place, then sorry you may have to walk away from the job, after all it's your name on the certificate, and should anything go wrong due to lack of any main bonding, it will be your neck on the chopping. block