Our builder has, much to my annoyance, fitted a mortice sash lock on a
new bedroom door and two bathroom locks on two other new doors which are
not capable of supporting unsprung door knobs. We have some nice brass
knobs to use.

I had absolutely no idea about this nonsense. I just wanted to slap a
nice brass door knob on the door, but no, some idiot decided that they
would make door locks with insufficient spring strength.

Anyway, I deduced, from the catalogue, that the unit fitted was a Union
"2295" 2 lever mortice sash lock: about the cheapest damned thing possible.

We therefore need to change it for a different unit with a decent
spring, but I've had to email Union to ask which unit to get, as the
catalogue isn't explicit as to which ones take unsprung handles/knobs.

Needless to say, the various units are all different sizes. There are
dozens of models. At least it's possible to get them with the same
backset/centre measurements, but the positions of the bolts, the sizes
of the faceplates, etc, are all different.

What's the likelihood I'll have to tell the builder he'll have to get
new doors because the old ones can't be modified to fit a different
sized unit?

Michael