If you choose to go with an independent installation professional, you can learn a lot here, www.french-patiodoors.co.uk/conservatory-doors, the cost will vary depending on the door design
Residential UPVC External Doors
Sliding Patio Doors Cost Guide
If you need to create the whole opening in the first place, it will cost a lot more than if you are just fitting new doors into an existing space.
To get a better idea of how much the prices for various sliding patio doors are, then you can see some average prices at this cost guide.
Fire Doors: Why You Need To Consider Fitting Them
It is important to make sure that your doors are able to stand up in a life or death situation like a fire, and therefore UPVC plastic is the way to go. Go here for Average Double Glazed Doors Prices.