-
This alternative has an 82mm backset which might be a compromise:
With a case depth of 101mm. How will that suit the dimensions of the stile and centre rail, considering there's probably going to be some damage done deeper than the case depth.
Filling the holes left for the original key and spindle will probably be a bit awkward and the plugs may come out due movement in the future.
Yes, the side-by-side arrangement was the one I meant by horizontal. I hadn’t thought about weakening the door like that so thanks for the warning!
This alternative has an 82mm backset which might be a compromise:
https://www.periodpropertystore.co.uk/ironmongery-c4/door-furniture-c22/mortice-locks-c25/imperial-locks-g5000-5-lever-mortice-sashlock-p5378
It’s an old’ish house and for whatever reason in my mind a handle doesn’t look appropriate.