Having french doors put in. Should I withhold payment untill after a building inspector signs the job off.

You should have agreed this with your installer before hand, but if you feel uneasy, regarding any problems, then arrange for the inspection as soon as the works completed. This way the fitter wont be waiting to long for his money, as hes probably already paid out for the doors.

normally we would a deposit for the ordering of the doors and then a final payment oncew the job is completed and passed