I will be fitting a 1500mm wide double glazed PVCu french door which
will be the only door at the back of the house.
So it needs to be very reliable when used as a single door, i.e. one
side left locked / closed, and the other side used as a single door.
It will be used a lot like this, since only double opening required in
summer as you can imagine. Can anyone recommend good manufacturers for
this purpose ?

By the way, I saw some cheap french doors in B&Q. Then I realised why.
There was a great big bar down the middle. You openened the two sides
and there was a great big post in the middle of the gap ! Who would
design such a thing ?
Simon.