Although not able to answer your specific question, an excellent place to watch is as the race comes up through the Brecons - according to the website, the top of the "mountain stage" is somewhere close to Storey Arms (a useful postcode is LD38NL) - this is on the A470 south from Brecon.
Apparently loads of parking, very scenic, and excellent biking roads (although not if you are on a push bike as lots of uphill bits).