Well, I know that 'Gog' is someone from the North (Gogledd) and Taff refers to someone - I thought - from the Taff Valley or surrounds. So, Splott, you use it just for The Valleys? (Of course, there's also another Taff, or Taf, river just west of here which flows into Carmarthen Bay at Laugharne.)

'Hwntw' I don't know. 'Tu hwnt' means 'beyond' so it might be the Gogs referring to 'those down there'. Also, there might be a connection with the fact that Welsh speakers from the Tawe Valley (Swansea and above) have a habit of injecting 'w' to the end of sentences, much as in 'Canadian, eh?' or 'Murican, huh?'

Another category within the Welsh is the "Down Belows" - the people in Pembrokeshire who live 'below' the Lansker line, roughly the A40.

sian