Sminthurus nigromaculatus is almost identical in appearance to the very common Sminthurus viridis. The only differences are in the colouration of the dorsal side of the sixth abdominal segment (two dark spots in Sminthurus nigromaculatus - no spots in Sminthurus viridis) and the number of setae on the subcoxa of the third leg (two in Sminthurus nigromaculatus - one in Sminthurus viridis).
There are a few scattered literature records for Sminthurus nigromaculatus, mainly by Peter Lawrence. However, the few specimens labelled 'Sminthurus nigromaculatus' in the NHML slide collection all have one seta on the subcoxa of the third leg, and Steve Hopkin remained deeply dubious of the validity of this species let alone its occurrence in the UK.
Cynicism is always to be encouraged, but Bretfeld (1999) lists Sminthurus nigromaculatus as a good species. In August 2009 Thom Dallimore collected some greenish Sminthurus from Freshfield heath Lancashire which had two spots on their abdomen, and (when checked by Peter Shaw) had 2 subcoxal setae on T3, fitting the description of Sminthurus nigromaculatus. In July 2012 Stephanie Bird collected Sminthurus nigromaculatus from Wisley heath as part of her PhD sampling. Sminthurus nigromaculatus appears therefore to be a valid UK native species. It may not be a coincidence that both collections were from sandy heaths. Bretfeld says it is a species of low vegetation in dry grasslands, and that some old papers about "Sminthurus viridis" may refer to this species so give erroneous information on chaetotaxy etc.