Hi Bryn - regret your picture too small for detail to be seen - hope you can take pix again and re-size with the first dimension maximum 700, and hope you're able to add a picture of the oval stamp.
I don't off-hand recognize the shape, although believe this green was a colour used by Wedgwood - when you say 'oval stamp', assume you mean a sandblasted backstamp, and not a label - Wedgwood used a variety of backstamps and labels over the years, and I think their oval design was a late example in their production range. Their early backstamp was the Portland vase design.