Greater Burdock vs Lesser Burdock?

Does anyone know if there is a major difference in the amount of root produced by the 2 different varieties. I'm guessing the Greater (Arctium Lappa) has the bigger root. More importantly is there a place in the UK where I could get some of the seed despite being known as a weed I haven't seen any where I live. I did buy some Greater Burdock off Ebay, but the seeds failed to germinate   .