Solved Mystery blue flower suddenly appeared

A gift from the birds maybe, I don't recall planting anything in this shady dry corner:

 

It does look like that thanks @mazambo , it's certainly nothing I grew so how it arrived there I've no idea. It is a beautiful little plant.

 

Just looked back through my spreadsheet and in 2013 I sowed RHS free seeds "Lobelia x speciosa, H., tubular two-lipped purple flowers, 1-1.5m.". I normally grow seedlings on in pots and plant when they get big enough, I have no recollection that these ever germinated. It must be a tough little plant for me to accidentally discard it and grow in less than ideal conditions.