On one trip doing VOR holds on the DTY VOR at 1800ft we gained a covering on the wings and fin that we were able to remove on the ground in the shape of the front of the tail fin. Personally I consider this severe!
I see. There are standard definitions of icing:
Trace icing is barely perceptible and presents no hazard to flight
Light icing is something that you can fly in for up to about an hour without significant hazard
Moderate icing requires regular use of de-icing equipment
Severe icing is defined as icing in which sustained flight is impossible even with regular use of de-icing facilities.
Severe icing should be reported to ATC and constitutes an emergency, in my book at least, and if it happened at Cambridge I'd expect to read about it in the Safety Committee minutes. What you encountered sounds like it fits the definition of light, possibly moderate, icing.
We'll have to agree to differ on the frequency of occurence.