Re-reading this, I see that the plan is to have trains from SVB alternatively going to Yate or Portishead (as opposed to Westbury).
P.46, incidentally, refers to a 3tph service from SVB to 'BTM▸ ' (presumably BRI» )... I'm not sure how all this fits in with the 4tph service from BRI to WSB» ... Confused? I certainly am.
I believed it was doable until I read this...