I think this is Dwarf Comfrey (Symphytum ibericum). Beth Chatto describes it as "impenetrable weed-cover" and Martin Crawford calls it "a fantastic ground cover". Certainly this patch on a nearby allotment is holding its own even against couch grass and continues to expand.

 

 This particular allotment seems to be abandoned and I took the liberty of teasing out a few root cuttings for my own allotment. Several sprigs I planted last year quickly formed a large clump -it spreads rapidly by rooting shoots.

 S. ibericum typically has yellow flowers in early spring and looks very much like Creeping Comfrey (S. grandiflorum). Having said that this form has red buds and white flowers that take on a blue tinge. The aforementioned Beth Chatto sells a variety called "Blaueglocken" which fits that description so perhaps that's what we have here.