How do you know what he "probably" meant, and how will you ever know? As for me, I'm going from the context provided by his sentence. Insidious implies stealthily, gradually, sneakily, infiltratingly, underhandedly, reds under the bed.... that's what Mail-Tories like you and Iains generally neurotically invoke when we're talking about commies/Militant/Momentum, etc. So give over, Nigel. Insidious fits a hundred times better and you know it. He seemingly doesn't. Isn't this fun?

I bet Davis wishes he'd never said Mad Max... Anyone for a chicken that smells of bleach?