Is it the single form that you want, the old 'pot marigold', Chiltern seeds are advertising a heritage variety

Calendula officianalis is the English marigold/pot marigold, you can get single or double flowers in the same seed packet, if you just want double flowers, you would be better off buying a named cultivar, Derek.

http://www.plantsofdistinction.co.uk/acatalog/A-Z_of_Flowers_CALENDULA.html#a1_211222

hi to every one who answered my question your replies are most helpful , thanks to all

I find that if you let calendula self seed they gradually revert anyway. If you are wanting them to use in tinctures or creams the calendulas work just as well. (Make the best hand cream around!)

Thanks Steragram thats another option i will try