Thank you for your help. I hadn't realized just what a potential future problem when enjoying it's flowering period. It's a hidden time bomb waiting to explode. Have now treated all the roots that are rapidly appearing with "Roundup weedkiller gel" . (Glyphosate 7.2g/l).
Here are some suggestions: https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/controlling-st-johns-wort.htm