For what it's worth the CAA Supplement to the R22 POH says:
Icing conditions must be assumed to exist when in cloud, or when the visibility at takeoff is reduced to 1000 metres or less in visible moisture, with a true air temperature of 0°C or less. Icing conditions must also be assumed to exist whenever there are any visible signs of ice or slush build up on any part of the helicopter.
It goes on to state: Flight in freezing rain or freezing drizzle regardless of visibility is prohibited.
That'll do for me