You need to tell your insurance company of EVERY mod you undertake, induction kit included...and yes they will most likely charge you more. Mine wanted an extra £50 just because I had a K&N cone fitted.
If you don't and they find out, such as after a crash you are VERY unlikely to be covered for your loss although they may cover 3rd parties.
It amazes me with all the boy racers running around with small, modded hatchback cars...chances are that most if not all, haven't told their insurance companies and hence are not covered....all well and good until they whack someone.
As replied in other posts about this before,i never tell the insurance about anything i do to mine and certainly will never tell em,dont we all reckon they rip us lot off enough
,if anything always act thick,e.g i dident know the car was modded,dont really know much about em. 
What you're doing is illegal fella - as you're telling them to insure a car that really isn't what you say it is.
Also, ignorance is no excuse, I'm afraid, as all policies state this somewhere (or words/meanings to that effect) - my missis works in motor claims, and if they decide you're not eligible in a the event of a claim, then you effectively HAVE NO INSURANCE, which means an investigatio (which you will pay for), court, fines, costs, possibly internment.
Seemples!
IF you find yourself in a position that you or a third party needs to claim on your insurance, you'll most likely find the insurance company WILL NOT pay out, and furthermore, you'll probably never be able to get insurance for a car again for years. It's a problem that the ABI are taking seriously.
So really, you're being the very irresponsible driver that we have to pay for on our policies.
