Hi Caroline
Thank you so much for sharing your experience and advice. I totally agree with you; the garden centre should have told me it was seriously invasive when I bought it. I actually had a member of staff with me, telling me which plants were the best for shade, and heavy clay soil, so they had ample opportunity to fully inform me. If I had known what would happen, I would never have bought this plant.
I will try and start digging, perhaps after some rain since right now, with such dry weather, the clay soil is like concrete! I think I will have a long fight on my hands from what you say.
Many thanks once again.