Confidence: It's likely to be this, but I can't be certain.
Notes: The plant has spotted leaves which pure southern marsh-orchid does not (except in the rare var. junialis which has bold unbroken loops on the lip). I'd say it is more likely to be a hybrid with a spotted orchid because of this plus the robust form of the plant and other small details such as the lateral petals being held more horizontally than would be expected for pure southern marsh orchid. I'm usually a bit wary of saying which spotted orchid is the other parent of plants like this, but in this case I'd go for heath spotted (D.maculata) as it appear to be quite fertile (there seems to be a group of similar looking plants)while the hybrid with D.fuchsii is almost completely sterile. Also I'd expect a more dissected labellum if it was a hybrid with D.fuchsii.