I am sure this is an Aubretia, but as to what one it is, as it has invaded is beyond me! So many synonyms and unresolved names!
Regards.
Neil.

Aubretia has four petals and your plant has 5. It looks like Campanula, possibly Campanula portenschlagiana or one of its hybrids like Campanula 'Birch Hybrid'.

Danita's right.

I must thank you for your help. I do have Aubretia in the garden, but this Campanula portenschlagiana is proving a rampant pest. My mistake on I.D. as it has grown everywhere,
I have a problem with Digitalis, as they too grow in the most obscure places and have to be weeded out, each spring. In the old vegetable garden as the pH is acid, wild Lupins are a major pest.
Thanks once again, for your most kind help.
Regards from across the Atlantic.
Neil.
p.s. Loads of weeding to do!

One of my Calla lilies.

Campanula portenschlagiana is very well known as a pretty but thuggish invasive.
By the way, it's Aubrieta, not "Aubretia" (a very common misinterpretation of the name).