I have been periodically searching and found a few references to the MTB frames, but really not very many so far. There is one other member on here who managed to find one too, but hasn't built it up yet.
I've seen a couple of people on forums mention having Pongo build one for them or a family member, a suggestion that mine could be made of 531c, that I haven't followed up to see whether that's probable and someone saying that Pongo them he made a few mountain bikes for Tim Gould and David Baker, re-badged as Peugeots, which sounds odd as far as I remember it was Roberts who did that.
I'll keep looking when I remember and if I find any hard evidence I'll certainly share it. If oanyone has any leads for me they'd be gratefully received.