Inspired by Robot Of Sherwood and contains spoilers as a result:
The story of Robin Hood is played pretty straight apart from the time-travelling space robots. The Doctor notes that it’s pretty far-fetched.
So... maybe that’s why the robots went for it.
Pick a well-documented period so they can blend in, but one nobody really believes happened as the stories say, so they won’t cause paradoxes. Recruit the ambitious and cruel Sheriff to steal all the gold and enslave the populace, which in turns spurs Robin and his band on to further action. The Doctor and Clara solve the robot issue, Robin takes out the sheriff and the minor paradoxes work themselves out.
An unlikely bit of history is the perfect time to hide. If all we know about Robin Hood comes from twice-told tales, use that as cover. Did the Zygons start the Loch Ness Monster myth or just hide behind it?