Hi Robert,
try the following link - there are two Class 20 plates and one for a Class 37 up for grabs.
http://www.gloucesterrailwayana.com/locoitem.php
The prices for the two 20 plates are different due to the higher priced one being a) In an ex loco condition and b) from RSH Darlington works.
Bear in mind that listed auction prices don't normally include the buyers premium so the above prices aren't too far off the mark.
Regards
Steve