Can you trash a card that you have already played, when remodelling? E.g. I play a village, then a Worksop, then Remodel - trashing the Workshop I just played? It says from your hand, but am unclear if there is a distinction between hand and play.
No, you cannot do this. Your "hand" always refers to cards that you physically keep in your hand. Cards that you have played this turn are in a different place, the "in-play" area.
On page 6 of the rulebook:
To play an Action, the player takes an Action card from his hand and lays it face-up in his play area.
Pages 9 and 10 also show picture examples of the different places cards might be; including hand, deck, discard, or in-play.