What happens to any Monsters left in the Outskirts when the Terror Level reaches 10. Do they remain there for the rest of the game and have no effect on play.

They return to the cup.

Does it actually say that in the rules as l can not find it.

It does not, no. It was clarified sometime later.

Can you please let me know where l can find the clarification.

Sticky thread titled Official Answer from Kevin Wilson on the first page:

" question: 1) When the Outskirts fill up and the Terror Track increases by 1, all monsters in the outskirts return to the monster cup. So when the terror track goes from 9 to 10 in this manner, all monsters return to the cup as well. However, if the terror track increases to 10 by some other means, there is no way for the Outskirts to be emptied. What happens to these monsters: do they remain in the Outskirts until the end of the game? Or are they returned to the cup like normal?

They should be returned to the cup once the terror level hits 10. "

Thank you both for your help.