Beautiful, beguiling, but very very tricky. Even the Plant Crib (BSBI) is coy when it comes to identifying this group, referring you to other monographs on the genus and to Stace. I believe there is a BSBI Handbook under construction but in most accounts the Eyebright, Euphrasia officinalis agg. , is usually not identified to the species, sub-species, or hybrid level. Essentially it requires an expert to come and see your plant in the field to make a confident stab at it.
They belong to the Orobanchaceae family (I think I've posted Common Broomrape if you are interested to search) which have in common that they are all parasitic or semi-parasitic on other plants. In the case of the Eyebrights the grasses provide the without-which-not.
The common name derives from their use in the treatment of eye conditions such as conjunctivitis and blepharitis, both as a decoction and a poultice.