Michael Kilpatrick wrote:
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.

Did you actually tell him (or the architect) what you wanted at the planning
stage?

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.

Perhaps you lack sufficient knowledge and information to know that there
are different types of locks and latches - and if you didn't tell the
builder precisely what you wanted fitted on each door, then the fault is
yours and you are the "idiot" that carries the blame!

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.

If the builder is not told otherwise, other than a simple mortise latch,
this is a standard fitting on a bedroom door - simply to give you some
privacy when there are children in the house.

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.

Why e-mail Union, when you could simply ask your builder to tell you the
differences between the different types of lock?

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.

You are simply making a mountain out of a molehill - almost all mortice
latches and locks will have sufficient spring pressure to operate a simple
door-knob setup - unless the knobs are rather heavy, and something will
usually still fit anyway with minor adjustments.

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?

Unless you told him exactly what you wanted at the planning stage (or he
fitted something contrary to your instructions), the builder is entitled to
charge you for any extra works - including the cost of new doors and locks.
As a matter of interest, all doors can usually be "modified to fit a
different sized unit" fairly easily by a skilled carpenter.

So why not talk to your builder about the problem, tell him precisely what
you require and then let him sort it out - in almost all problems of this
type, new doors are not needed [1] as locks can generally be found to suit
the housings.

[1] With the exception high quality of polished doors, as any making good
can be difficult to hide.