If you don't declare it and you have a crash your insurer max declare your policy void. Not only would this mean no payout you would also have to declare on subsequent policies that this had happened or those policies could also be declared void. Is it worth the hassle for a few saved quid now?

We didn't have to provide engineers reports for the 2litre conversion we did on an XR2. In fact he got a tug because the police were following and a check came back as Ford Fiesta XR2 2litre. The copper thought it was off so stopped him and nearly fell over when the bonnet was opened.

Incidentally I found the admin charge more than the premium hike when I chipped my car