Intrigued about the Sunday medium/big name, someone 'very popular' according to someone on Efests, and doesn't sound like a forum favourite so I think that rules out Idles, at least for now. Probably a name for the younger crowd?
London Grammar is a good shout.
Noah Kahan is still available but hardly qualifies as a medium name anymore. Doubt his management would let him play under The Offspring. Would shift tickets though.
It'll be Oscar and the Wolf, or Joost, no doubt.