I just created a dummy DB with 3 tables. 1 testV that has dummy info about
vehicles, 1 testD that has dummy info about drivers, and 1 testDV that lists
what drivers drive what vehicles. 1 form that lists all the info about a
vehicle, 1 subform that lists all the drivers for the displayed vehicle and
the detailed info about that driver. in the main form I can enter a new
vehicle as well as look at existing vehicles. in the sub I can add existing
drivers to an existing vehicle or a new vehicle or I can add a new driver to
an existing vehicle or a new vehicle. when I add an existing driver to a
vehicle a new entry is entered into my testDV table, when I enter a new
driver into the sub that new driver is added to the testDV table as
registered to the vehicle that was displayed when I entered the new driver as
well as being entered into the drivers table at the same time. I choose an
existing driver via a combobox of existing driver ID's. when I choose an
existing driver the rest of the fields auto-populate with that drivers
details. when I enter a new driver I just type that driver a new driver ID
into the combobox. If this is what your looking for then I'll tell you how
its done. (considerably simple) if it's not what your after then let me know.

So everything is working fine now except when entering a new driver a check
needs made to see if the driver being added to a vehicle isnt already
assigned to another vehicle. you might need to look to some VBA for this one.
like maybe on the AfterUpdate of your subs driverID field you could put
something like:
If Dcount("driverID","VehiclesDrivers","driverID='" & me!driverID & "'")>0
then
   msgbox "this driver is already assigned to a vehicle."
   DoCmd.DoMenuItem acFormBar, acEditMenu, acUndo, , acMenuVer70
end if

or something similar that checks and stops the adding of a new driver that is
already assigned.

or if you just want a combobox to only show drivers that are currently
unassigned to any vehicle then you might put some SQL on the combobox's row
source like this:
SELECT drivers.driverID FROM drivers LEFT JOIN vehiclesdrivers ON drivers.
driverID = vehiclesdrivers.drvierID WHERE (((vehiclesdrivers.drvierID) Is
Null));

Thanks Dirk and Kelly for your posts.
Just to confirm, I am looking for a way to see if a driver already exists as
being assigned to another vehicle, so that the user will not input the same
driver twice but instead just select the driver and the driver then will be
assigned to multiple vehicles (and will populate the driver info fields in
the sibform for this particular vehicle.)

I have tried Dirk's idea of removing Drivers.DriverID.  Here are the results

Yes it does solve this problem in the short term.. but I now cannot save the
form or database without the whole of Access crashing!  Unbelievable.  This
has never happened consistently before and has just crashed the last 4 times
I have tried this.   Some of the VB is missing when I reopen- I have not
checked everything, but the command buttons on the main form have lost all
their VB and now do nothing!  when I re-open Access so I have to go back to
the backup version I saved at lunchtime - fortunate I made a backup.  (I am
resisting the very strong urge to say harsh words about Microsoft!)

The combo is in the header section of the subform.  I am a bit confused as
to why it does work anyway because in design view, the combo says for its
recordsource VehiclesDrivers.DriverID, "Error- No such field in the field
list."  And there isn't!  Yet the recordsource query for the subform includes
the VehiclesDrivers.DriverID field!    Despite this, in form view it is
working!   Am I missing something here, or is this very strange indeed?

Here's the recordsource for the subform:

SELECT VehiclesDrivers.VRM, VehiclesDrivers.DriverID, Drivers.Surname,
Drivers.[First Names], Drivers.DOB, Drivers.Sex,
fncDriverWarnings(VehiclesDrivers.DriverID) AS Warnings FROM Drivers INNER
JOIN VehiclesDrivers ON Drivers.DriverID=VehiclesDrivers.DriverID;

Anyway, it works short term (until I need to save, when Access crashes,
negating any benefit and creating more work.)  Drivers.DriverID was in the
record source for the subform because I had been using Drivers.DriverID on an
"edit driver profile" button on each record, to open the corresponding record
in a "Drivers" form where users can edit warnings for the driver.  As
Drivers.DriverID had been removed from the recordsource, I have instead been
using VehiclesDrivers.DriverID to find the correct driver, which works.

Has anyone else experienced anything similar????  Does this all stem from
the peculiar issue in the relationships window, where Vehicles is still not
showing as being in a relationship with VehiclesDrivers, even after I create
the relationship, close the window and say yes to saving the changes in the
rel. window?  I still haven't got to the bottom of this issue which worries
me, Microsoft!

just for the record, the "Add Driver" button sets
Me.AllowAdditions=true then opens a new record using the standard code.. and
it works fine.  All of it does til I save!

Please please help if you can!  I have spent all afternoon on this.