I would clean the lintel and keep it natural stone, paint the frame black and the door F&B vardo.

For anyone else reading this thread, you can now get aluminium internal sliding doors either frame or surface fixed. These come fully glazed, in the steel look and even as frameless glass sliding doors. Happy to help with product advice and suppliers if you'd like to get in touch. Hope this helps Kind regards, Nick

You need to consider the space into which your doors open to. Ideally opening out of the new extension. If its not too spacious, or might cause things to be crammed, only then can you go for bifold. Bifold will really only come in handy if you have a problem with space. Do you have pictures or a plan you can share?

The thermal qualities of new ones would be preferable. When I had new patio doors (sliding) I couldn’t believe how much cosier the room is now compared to the old 1980s one. Your examples are beautiful. I recently had one of the large double glazed panes changed to accommodate a cat flap (safety glass cannot be cut) and I was quoted £370 - £650 for one pane. I chose the cheaper quote but just to let you know of possible costs to change the glass.