Vipers bugloss
I noticed this plant last year, but it had not flowered and since I do not remember buying it I thought it was a scabious or a knapweed judging from the leaf rosette. I was amazed when it threw up these amazing blue flower spikes in June this year. I really don't remember buying these as plants and it is highly unlikely that they would have seeded since it is such a rare plant in the wild (restricted to a small number of sites in Cornwall I think). However, it might be an ornamental which has self seeded or my memory may be at fault. Of the two I think that that the latter is far more likely!