They are quite narrow. The existing frame - which incorporate two big windows and two little opening windows as well as the opening doors in the centre, is in place, and in good nick. They are the originals in the house (20s / 30s semi). And yes - each door is 535mm.
Of course, they were made long before metric - and are taller than most I have found so far on the 'net - will have a look at laporte!
I need a new back door, too - and it seems it is thinner than modern doors - aaargh!