Dear Philip,
I looked up your pottery mark in my Kovel's New Dictionary of Marks, and there are two listings for Royal Cauldon.
The earlier mark that is listed dates to the 1930's, and they were reissues of F & R Pratt designs.
The later mark sounds close to yours, and dates from 1950 through 1962.
The Cauldon Potteries Ltd. was taken over by the Wedgwood Group, so perhaps you could try to search the values of your plates using some Wedgwood sources.
Sincerely,
Linda Linda Slocum
www.worksbyhand.com 
