Confidence: It's likely to be this, but I can't be certain.
Notes: This is the most widespread of three similar looking moths on p254 of Sterling and Parsons. Crambus silvella does not appear to be found in this part of the country but Crambus uliginosellus would be possible both with regard to presence in area and habitat of damp grassland. I have identified the moth in the pictures as Crambus pascuella because it appears a fairly dark moth (C uliginosellus is a pale species)and the longitudinal white streak does not appear to follow the costa to about one half as in C uliginosellus. Also the shape of the soft brown coloured triangle in the clearly delineated white triangle resting on the costa near the termin is more in keeping with C pascuella.