Confidence: It's likely to be this, but I can't be certain.
Notes: Very difficult to deal with flowering material, especially without measurements, but to me this looks more like M. sylvatica, and quite possibly var. culta (the usual garden version of the species). M. arvensis ssp. umbrata (= var. sylvestris) is relatively large-flowered, but tends not to have as showy an inflorescence as this. However I have looked at a lot of herbarium material and until the plants are in fruit it can be difficult to be sure. Alan Silverside (author of BSBI Plant Crib account of the Mysotis arvensis subspecies).