First I rehydrated some yeast and plated it out on WLN agar. In the main the colonies where very white, though a few colonies with a green tinge were in there too. The white colonies looked very much like lager yeast, so it appeared the man from Surebrew was right. I was after better evidence than just the look of the colonies though, so I separated out the different strains as best I could and incubated them at 37 degrees C. Lager yeast is unable to grow at this temperature so if the white coloured strain didn't grow the results would be confirmed.
That's not how it worked out though, as two days later I found both strains had grown well at 37, though for some reason they'd both developed more colour.
So I've found no evidence that there is lager yeast present. Nottingham Ale Yeast is indeed ale yeast, though it does appear to be a mixed culture. My investigations continue.