Personally I would go for a universal handset as they have separate mic and speaker adjusters inside the handset ….i have used the comelit one before ….see below anything else is a trial and error
You will need to try to knock out the brick in the loft on the inner coursework ( depending on how high it is from the ceiling ) had to do it many times on 1960s houses to get bell or camera cables up or drill up at the correct angle..