I have been following news of this bird on Birdguides and BF, and also received emails and phone calls from friends who have been to see it. From the excellent photos I have see this is definitely a Marmora's, although it was originally put out as a "possible".
Several observers have said that the bird is definitely collecting nesting material which suggests that it has a mate and might possibly even have copulated. But, with what? As far as I can see, no-one has commented on what species the mate might be - the chances of another Marmora's of the opposite sex in the U.K., let alone Gwent, must be nil.
Or is this just hormone-driven, compulsive nest-building?