a. Have you been given an itemised invoice
b. When you say you only got the wetback installed - who did the new fire?
c. Have you gone back to the company to discuss the invoice?
d. Do these people belong to any professional body - e.g. Master Plumbers - and if so can you contact them?
e. Were you given any information along the way to suggest there were problems?
f. Did the original quote have any exemptions that were not allowed for that may have come in to play?
Always go back to discuss the invoice if you are not sure sure - ask some questions around why two people where sent for a one person job - and ask if you have been invoiced for the second person that was not necesarry, and if so why?
As long as you are reasonble then you should be able to discuss this, and this is always the first thing to do - failing this working you can always go to small claims, but only once you have exhausted the friendlier options first. When you call up the business, make sure you are talking tot he boxx.