I have a 12 in platter mason's ironstone china elgan england c4666 what is it worth?

According to www.thepotteries.org/features/ironstone.htm, an Ironstone China plate can have a value of anywhere between £20 and £300 depending on the following factors:

• Factory
• Rarity
• Mark
• Date
• Condition

However, it is possible the value of Mason's ironstone china is more likely to be between $100 and $1500. If you are aware of any auctions taking place, these will give you the best indicator of the going rate for Mason's ironstone. Beware the impact of market forces.
If you are a collector of Mason's ironstone, then there are a number of enthusiasts you might want to get in contact with, as well as some useful literature that might help answer any queries that you might have. There is currently a Mason's Collectors' Club in England with the following address:
The Secretary
Mason's Collectors' Club
c/o City Museum and Art Gallery
Stoke-on-Trent
ST1 3DW
United Kingdom

According to www.thepotteries.org, a North American society is in the process of being formed.

• Ironstone China

Ironstone china was first produced by Charles James Mason in 1813. It originated in Staffordshire, England and began to be exported widely from the 1840s onwards. Most ironstone china is mainly white and is decorated with floral patterns, fruit or scroll patterns in colors such as black, blue, orange and red.
Original ironstone china produced by Charles James Mason should bare a mark that says 'Mason's Patent Ironstone China', with a picture of a crown above the mark.
There are a number of useful books about Mason and Ironstone, such as 'A Catalogue of Ironstone China' by Ashworths, 'Mason's China and the Ironstone Wares' by Geoffrey Godden and 'Mason's Porcelain' by R.G. Haggar.