Agree with gasman. I will cost you about�120 through the council. But you do need to show a FENSA Certificate if you are ever going to sell your house. The buyers solicitor will ask for one.
If you have a Non Registered fitter make sure he uses Low Emission Glass, has toughened glass where required and has fire escape hinges and trinkle vents if required.
Also theoretically the company should offer you an Insurance Back Guarantee, which you do not have to take up. This is a Guarantee that things will be fixed in the event of the installer going bust. However my opinion is it is not worth the paper it is written on Rgdsd Al