Iains: Personally I expect this division, coupled with the invidious advance of momentum, to totally destroy the labour party.

Steve Shaw: By the way, the word you were looking for was "insidious."

I think Iains probably used the word he intended:
invidious: (of an action or situation) likely to arouse or incur resentment or anger in others.
Insidious: proceeding in a gradual, subtle way, but with very harmful effects.

It may be that Schoolmaster Shaw knows what Iains' intended meaning was better than Iains himself, but I very much doubt it!