Thanks for all of your replies. I have had a look at all of the suggestions and am certain it is a Salvia guaranitica. Possibly Salvia guaranitica (Blue Enigma). I already have some hardy and half hardy salvias in my garden, so no wonder I was drawn to it. Just googling now has opened my eyes to the varieties.

I forgot to tell you where I was in the world when I spotted it - the beautiful county of Shropshire in the south of England.