Well, I rang up my insurance company, Direct Line, to tell them about my K&N air filter. They have charged me around £35 (can't remember the exact figure), which includes a £13 admin fee. I explained the filter in detail to them, told them it wasn't an induction kit, might give a 2% bhp increase, if that, but they said that was what I had to pay. And I've only got a few months left on my policy.
Well I'll definitely be switching coming renewal time ! :tsktsk: