Thanks for your responses!
It is one thing departing with ice on the wing, and quite another to get a slight accumulation of light frost during flight (and to some degree - not much you can do should about it when already airborne).
The argument to stay on the ground if icing could happen is a valid one, but what if it wasn't forecast, and you're arriving a little later than expected from a far away destination?
By very very light - I mean less than 0.5mm, patchy frost by the way, only visible upon very close inspection, not a wing covered in 1+cm of frost / ice!
Thanks for your responses!
Alex