My mate had a fiesta 1.1 with a RS1800 lump in. Some old duffer in a Volvo drove up the side of him and matey got nicked for having no insurance. It's called non-disclosure as in the event of an accident, is much the same as having no insurance. Now he can't get insurance at all as it's recorded on the insurer's database.

Here's an example which happened to my ex's brother. He had an XR3i with a RST lump in. Smashed head on into someone who broke her neck and is permanently paralysed. Normally his insurance would pay for the costs of medical care which the woman needs for the rest of her life but as he didn't tell his insurers of the engine change, they didn't want to know. The woman is now suing him for a high 6 figure claim. Be warned- it's not worth it.