I have dealt with cat c and d cars for years and no cat c only shows on the log book but this only happened about 8-10 years ago ( I think time does fly) before that it didn't show I have also bought catc cars which haven't needed a vic check so not on the log book but still cat c when you hpi .
I always tell people what it is to avoid confusion and the loss of a sale but was always told that by law no one has to tell you only if you ask ! I know traders that sell cat c and don't tell customers (I don't know how they get away with it) hope this helps