The previous owner should take the plate off before he sells the car or be in the process of transferring it off the car when you take it away.
If you have bought the car without the previous owner doing neither of the above then technically the plate is yours. If he wants the plate back it's down to you to do the paperwork etc to transfer the ownership of the plate back to him and transfer it to his new vehicle or place it on retention.
The only other way is if he gets you to sign something stating the plate is not included in the sale and you agree to transfer the plate back to him (more of a gentlemans agreement).
see Section 10 on:
http://www.direct.gov.uk/prod_consum_dg/idcplg?IdcService=GET_FILE&dID=84243&Rendition=Web
10. How can I safeguard my
entitlement to a registration
number?
Entitlement to a number is effectively the right of the registered
keeper of a vehicle to
apply to have the number transferred to
another vehicle (either his own or someone else’s).
The keeper is also entitled to apply to have the number of his
vehicle put on retention, that is to say, take the number off the
vehicle and put it on hold pending re-assignment to another
vehicle. It is important to note that although the keeper may
apply, the application will only be granted if all the conditions
relating to the retention and transfer facilities are satisfied. You
can help to safeguard your entitlement, by ensuring that your
vehicle is able to meet these conditions. You will find more
information about the conditions in section 15 of this leaflet.
If you no longer have the vehicle which properly
displays the number (ie it is in someone else’s
keepership or has been scrapped, broken up,
destroyed or exported) then your entitlement to
the number ceases.
