Could anyone help me to put a name to this little Campanula please. Clumps of it are dotted around my garden and are full of flowers and regardless of the attention it doesn't get it continues to flower. The clumps are about 15-20cm high, the leaves are small, and it spreads quite well, but I wouldn't regard it as a 'thug'. The flowers are a beautiful shade of lilac/blue that is difficult to explain. The picture below doesn't do justice to the proper colour.
I have ploughed through my copy of Graham Nicholl's book 'Dwarf Campanulas and Associated Genera' and the closest I can come to is C. portenschlagiana