If your requirement is only the French doors, suggest you go to a local company rather than a big national one - who are more geared up to changing lots of windows / doors. You should find several in Yellow Pages.
Glazing jobs involving replacement windows and doors are covered by part of Building Regulations (in England & Wales at least). It means that your job must either by done by a FENSA-registered company or a Building Regulation application is made at the local council. FENSA companies will self-register your job and give you a certificate.
This was done to ensure that replacement glazing is done to an appropriate minum thermal standard. You can find a FENSA-registered company here.
http://www.fensa.co.uk/homeowners.aspx