Do you have any Davenport?
My mum bought 6 Davenport Botanical plates for £5.

I couldn't tell you how much they are worth, ebay only brings up one result.
http://www.ebay.co.uk/itm/Original-19th-Century-Davenport-Bone-China-Hand-Painted-Cabinet-Plate-NR-/181695082920Blue printed mark "DAVENPORT LONGPORT" in uppercase letters.
From what i can gather, this underglaze blue printed mark was used 1837-1860. My guess based on the pattern number (1580), is that it dates around 1837, but i could be wrong.
I read somewhere that the prince regent visited the factory, found that the quality of the porcelain was up there with Sevres, and ordered a whole batch from them. These plates are of lesser quality i think.