It wasn't - the white-edged costa indicated either C. lathoniellus or C. pratella, and the relatively broad central white stripe suggested the latter. The key character lies in this central white stripe, which has an angle on its costal edge and is straight-edged in C. lathoniellus. I have arrowed this feature on one of my photos, because it's not immediately understandable when read about in the books, and have included a similarly arrowed photo of C. lathoniellus for comparison (thanks to Ian Morgan for his image, as I didn't have one of my own.
Crambus pratella from Dingestow and Crambus lathoniellus from Llanelli
arrows show angle on costal edge of the white streak on the C. pratella
and the straight equivalent in C. lathoniellus
There are two confirmed VC35 records of Crambus pratella: both made by Richard Clarke at Ochrwyth in summer 2013. There isn't really any suitable habitat - unimproved, short dry grassland - at Dingestow and I'm sure this individual has wandered from further afield. The Micros book suggests that some/most southern British records are migrants from Europe, and my Udea ferrugalis indicates there was migration going on last night, but I wouldn't be surprised if its origin was somewhere in SE or SW Gwent.