...at least that what it looks like to me, but I'm happy to be corrected.
This evening I eventually got round to looking at a specimen of another unfamiliar species I collected on the weekend, from the same clump of tree ferns where the Ptychomnion grows in the Wallace Garden at NBGW. Apparently these ferns were imported from New Zealand about 5 years ago. I'll post a few microscope images later, but here's my rather paltry specimen to give the general impression (for scale leaves ~8mm). Cell structure matches the illustrations HERE perfectly...fingers crossed I've not overlooked something native!