Thanks for the replies guys! It's good to have expert help on hand like this
It's interesting to find it where we did, as we didn't see that kind of moth when we had a team in to do a moth survey just 3-400 metres away, although that was in July and it seems that the Pale Tussock doesn't fly then. Different kind of trees as well though - the survey area was mostly sweet chestnut, birch and oak.
Mike