Strollin' Johnny, there IS a lot of bad workmanship around, but in this case it is not necessarily the guy's fault.

Who decided that the boiler needed replacing and why?
Who defined the scope of the job?
Who connected to radiators incorrectly? Was it this guy or the one who first installed the system? and so on.

There are so many unknown factors that it's difficult to know who is to blame - So why automatically blame the plumber.