Hi Antony,
Yellow readers are positioned at some key points within Stratford, but it can be easy to miss them if you’re not sure. There are a couple at each end of the platforms shared between Central line and Elizabeth line trains. There are also some on the way to the Pudding Mill Lane DLR platforms, and there’s one in the Jubilee line concourse against the wall.
At Tottenham Hale you would need to exit and re-enter, although most people doing that will be switching to the Underground and you have to go through both gatelines to do that anyway. Farringdon has validators at various points between the Thameslink and Underground platforms, as well as a couple near the top of the escalators down to the Elizabeth line. Finally, at Finsbury Park there are several validators in one of the subways under the NR platforms. I think it’s the higher one, but I may be wrong.