How to Prevent Timeslot Overlap
You can only achieve such validation at the database level (when not using forms and VBA) using a CHECK constraint

ALTER TABLE Bookings 
ADD CONSTRAINT NoDoubleBookings CHECK(
       NOT EXISTS(
             SELECT 1 
             FROM Bookings b1, Bookings b2 
             WHERE b1.BookDate + b1.BookStart &lt; b2.BookDate + b2.BookStart 
             AND b1.BookDate + b1.BookEnd &gt; b2.BookDate + b2.BookStart
       )
)


More details on adding check constraints to Access databases can be found here