>Having, yet again, fallen foul of a partial build overwriting the
>source directory because the required $OBJ directory hadn't been

That's a makefile problem, though perhaps one that is tricky to solve
if you don't have a nicely constrained build environment.
Cookie files are generally evil, but I use one for this.
A make obj from top level will create a .obj.done, and virtually all
other top level targets depend on that (there are some messy details
to get this right btw).  And by doing cvs updates via 'make' you can
help ensure that that cookie file is removed whenever you update.

>created, I wondered if there were any reason why make shouldn't
>create the object directory when MAKEOBJDIR is set in the environment.

Yes.  This would likely break far more than it fixes.  

--sjg